Warding:
You really have 3 major zones that you want to ward outside of laning.
Defensive:
These wards should be in the jungle just in front of your tower line. Typically, that's either blue brush or the line bush that enters near mid, the line bush by raptors or the curved bush on the side just in front of your base, and the triangle bush near bot/top lane or the bush by red buff. This ward set is best when you're behind, and should give your team time to rotate on a split-pusher and should have you avoid getting flanked.
Neutral:
Warding the bush behind the enemy red buff, the bush in the river on blue side, and a ward that covers Dragon/Baron, depending on your side will usually give you vision near the river, catching people trying to rotate on you, and generally making seiges safer. You can rotate the Dragon/Baron ward to triangle bush when sieging blue bottom first tower or purple first top tower.
Aggressive:
If you're already ahead, a ward at the bushes near enemy red, enemy blue, and a ward either in the straight bush near raptors or a ward middle lane as far up as you can get it will set you up the best for picks as you're pushing further in, and even if they are aware, it will usually slow down their rotations as they search for safer paths around.
Generally, if you have a lot of enemies that can come in from bizarre angles, try to get your team to ward one of the major spots so you can keep a ward to help keep vision on the weird angles.
Confusion:
Generally, the thing about the confusion is best solved by having a plan and telling your teammates the plan. Make sure your ADC at least has vision before you roam out, and a failed roam that communicated should at least rarely end badly. One thing to keep in mind, though, especially if you're roaming after your lane is behind is that an extended roam can pull a lot more pressure on your laner than you can hold off. Make sure that you can help against their support and one other roaming in if you are elsewhere for an extended period of time.
Extremely important. If you fall behind 3 levels and haven't died a million times, you're getting too little out of your roams. Stay with your ADC, get some experience, and see if you can keep pressure by your presence and threat on bottom tower instead of being in lane.
Harassing
In general, melees don't harass, they zone. It's important to gauge where your ADC/Jungler are as a melee support. If you're in a position where nobody can follow, you lose a lot of the control that you can put forth. The threat of a Blitzcrank grab or a Leona Zenith blade is much larger than the damage they can throw, and wiffing on your engage ability means that you're giving a lot of free damage. I've been the Lulu or Sona on the other side of that matchup, and missing those abilities means that I actually have 10-15 seconds to poke you down and zone you. If they don't keep the lane bushes warded (and remember, that takes both the ADC and the support working together before first buy), that's where you can go to minimize poke and keep a lot of pressure up. As a hint along those lines, trinket wards last for 60 seconds and have a 2 minute cooldown. If you can put up pressure right when that ward fades, a lot of ADCs are going to back away and not get that ward down. That gives you 60 seconds where they can't see where your skill shots are coming from and they can't auto-attack you.
If you still don't feel comfortable with that thought, play Morgana or Nami and just keep an eye on how your ADC reacts to an enemy getting snared/stunned. That should give you a very good sense of how much room you need to leave different ADCs to react. If you watch that, it should make melee supports much easier.
You do have to remember that as a melee support, you will take damage. In return, you just need 1 good combo to force summoners if not a kill. If you can still make the play, do not panic if you take a couple of autoattacks.
Champions
I think nearly any character for support can have good results. If people wanted really generalist picks for support, I would suggest Thresh and either Lulu or Nami. In general, most of the supports that are played can both give their team a lot of utility and patch up a hole in a team comp. With that in mind, I would want Morgana or Annie as supports that are a secondary damage threat, Janna, Lulu, or Nami as a pure support that can protect 1 person like none other, Braum or Thresh as a tanky protector, and Blitzcrank or Leona as tanky engage. I do think other supports are awesome in their own way, but these will cover your bases efficiently.
I would say if you just wanted to keep going as Janna and Leona, I think with practice, you can get pretty far, and if you struggle against a certain support, play them a few times.
If you want, you can send me a friend request, and when I'm on, I'd be willing to watch one or two of your games and see if I can give you help.