I am a silver II support janna main. I might be able to help you a bit.
Find A Support (or two) That Matches Your Playstyle
Outside of maybe jungler, I feel that support has the highest champion diversity. There is a support that fits just about any playstyle for your teams needs. I can go over a quick overview:
Alistar is a good support that excels in hard cc, tankiness, and has a small heal. Alistar is a great pick when you have an active jungler since he can tower dive easily and set up ganks in a breeze. In the right hands, he can keep several enemy champions off your adc or engage a teamfight if needed. He is, however, very vulnerable to poke and has a learning curve. It is essential that you can do his q-w combo. He is a good pick against other all-in supports such as leona or thresh.
Annie is a high risk-high reward champion. She is one of the few supports capable of killing the enemy adc single-handedly! Her ultimate (tibbers) grants her an enormous power spike at 6, which allows her to snowball the lane for the rest of the game if used properly. She also has a ridiculous AA range and can convert to a second APC if she gets ahead. She does, however, lack any sustain for the ADC and is very squishy. She is a good pick vs squishier supports like sona or janna.
Like annie, he is a high risk-high reward support. His entire kit centers around his rocket grab, which can single out an enemy champion. Combined with his knockup and silence, the victim almost always dies or has to burn a summoner spell. He is also fairly tanky. Being able to land his grab is everything though. Compared to other supports, blitz doesn't offer as much without his grab and becomes susceptible to poke and can have his adc zoned quite hard. He also lacks any form of sustain. Blitzcrank is best against immobile, squishy supports such as soraka or sona.
Braum is a tanky, cc-filled support that excels in damage mitigation. His passive serves as a heavy deterrent to engaging on your adc and can ensure burned summoners or flash single-handedly. He has average poke and a slow through his q. Along with his ult, braum provides pretty good peel throughout the game. His shield is capable of shielding his allies from lethal skillshots or autoattacks as well. The biggest problem that he has is that he is fairly suspectible to constant poke and harass since his skills are on higher cooldowns. Braum is good against all-in supports or supports with heavy cc skillshots such as thresh, leona, or nami.
Fiddlesticks is a support that offers a lot of hard cc, good harass, and a deadly ultimate. In a small skirmishes, his fear can completely turn the tides in his favor. His silence and fear make him good at interrupting channels too. Proper map awareness, enemy vision knowledge, and team capitalization are required for his ultimate to be successful. His biggest weakness is his squishyness and lack of sustain for his own adc. Fiddlesticks does well against supports that lack good peel such as sona or annie.
Janna is a highly mobile, defensive support that is known for keeping enemies off her carries. Her trade-favoring shield and aoe-knockup/interrupt tornado make her a versatile lane in both peeling and kiting. She has a reputation for being difficult to gank due to her immense peel. She can completely reset teamfights, seperate the enemies carries from tanks, or heal her whole team with proper use of her ultimate monsoon. Janna does, however, struggle against constant poke and has arguably the smallest damage output amongst supports. Her cooldowns are longer and proper timing of skills makes or breaks her. She is a good pick against all-in supports that need to stick to your carry such as leona or alistar.
Karma is a heavy poke champion that also excels in kiting. Her mantra+Q can cause an overwhelming amout of damage early on. Her speed boost shield makes it hard to lock onto either her or her carry. Her bind serves as a warning to stay away from her or her carry as well. Proper mantra usage allows her to be flexible with her abilities so she can do extra damage, speed up her whole team, or tank damage. Karma, however, lacks a game-changing ultimate and falls off in damage if she doesn't get ahead. She is a wise pick against supports that cannot face the constant poke such as alistar or janna.
Leona is the opposite of janna in that she excels in offensive engagements and absorbing damage. She is an all-in support with excellent zoning and an overwhelming amount of cc. She is really hard to kill and can engage fights very well with her ultimate. Her passive adds to her already scary burst and she can snowball a lane very quickly. She does, however, lack any sustain and she is prone to falling behind if she doesn't do well in the laning phase or is overwhelmed by poke. She also lacks a true escape so must commit to a fight she starts. Pick leona against squishy supports that lack good peel such as soraka or sona.
Lulu is considered by many to be the jack-of-all trades support with heavy harass. She can do a bit of everything and works well with just about any adc. She has an aoe slow with extendable range, a speed boost or polymorph, and a shield or poke/vision ability. She has good peel and poke throughout the game. Her ultimate is highly versatile in that it can be used on tanks to improve engagements, or on carries to provide a strong peel. I consider lulu to be the best teamfight support overall. Unfortunately, lulu is very mana hungry and squishy while lacking true sustain. This makes lulu great against supports that can't cope with a lot of poke or rely on successful engages such as leona, janna, or thresh.
Morgana is a very safe pick for a support that doesn't really struggle against any matchup. Her bind has a long range and can potentially lock down an enemy for 3 seconds forcing summoner spells or death. Her shield can protect you or your carry from life threatening cc such as a rocket grab or aqua prison. Soul shackle allows her to peel for her carries or extend the cc in a teamfight. She does, however, lack the sustain of other supports and is reliant on proper timing of her shield and skillshots. Morgana doesn't really struggle against anyone, which makes her a good pick when you don't know your opponent.
Nami is another contender for most versatile support that excels in aoe cc. She has great peel or lockdown in both aqua prison and tidal wave. Tidecaller's blessing amplifies her or her carries damage output, while ebb and flow serves as both poke and sustain , which favors trades. Nami does, however, struggle with never ending mana problems early and is very squishy. Landing her bubble makes or breaks your nami play as well. and her cc is more delayed compared to the instant peel other supports feature. She is good against lanes that struggle against cc, sustain, or heavy poke making her good against janna or alistar.
Nunu is a classic support that excels in controlling attack speed and tanking. He is very tanky and can self=sustain at little cost. His snowball can serve as a mini exhaust while bloodboil makes hypercarries like vayne or kog even scarier. His ultimate provides aoe teamfight slows and damage. He struggles against poke on the adc and strong peel. Pick nunu into someone like annie or taric.
Sona is a support that is all about boosting her team through her auras and winning trades. Her hymn of valor amplifies ally AAs and pokes well, her aria of perseverance heals and shields, while her e serves as a team speedboost. Crescendo is a game-changing ultimate that stuns everyone in a line, which is useful in just about any situation. Sona is, however, very squishy, mana hungry, and lacks peel outside of her ultimate. Pick sona into lanes that trade poorly and struggle against harass such as janna or braum.
Soraka is the queen of sustain in the support world that none can top. She has a spammable, high value heal (costs hp) and a global heal that removes grievous wounds. She has decent poke with starcall and a good aoe denial tool with equinox. Equinox also helps weaken dive attempts and interrupt channels. She does however, have little sustain if she can't land starcall, is squishy, and struggles against all-ins. Soraka is a good pick in lanes that rely on poke to succeed such as karma or lulu.
Taric is a classic support that is a bit dated. He excels in his tankieness, targeted stun, heal, and auras. His dazzle-radiance-shatter combo is still potent and makes him a force to be reckoned with. His passive armor auras make him and his partner more resilient to the ADC's damage. Dazzle serves as a good targeted cc with its own zone potential. He struggles against heavy magic damage poke and aoe cc in general. Pick taric against all-ins pitted against a blitzcrank or leona.
Thresh is a scary support with a versatile kit--capable of engaging or peeling. His death sentence spells death combined with flay and the box. His lantern makes escaping bad fights a breeze or bringing an ally to a fight. He is also pretty tanky with AAs that hurt. His drawbacks are his reliance on landing skillshots, long cooldowns, and struggle against heavy peel. Taric is a strong pick against a sona or nami.
Hope this helps