If you've been gone 8 months, then there are a list of champs you may want to gloss over as they've received reworks of varying sizes. Since the last season also would have ended in that time, Riot took a look at some more basic aspects of the game and adjusted/added to them. So in short...
GENERAL CHANGES
The newest champs added to the game are Illaoi, Kindred, Tahm Kench, and Ekko
Graves, Corki, Quinn, Kog'Maw Caitlyn, and Miss Fortune were all reworked to some degree that I ranked approximately from most changed to least changed.
Mordekaiser, Darius, Skarner, and Garen were a little before them in the same pattern.
Then before them was Fiora and Gankplank.
In Riot's attempt to speed up games, turrets fall faster than they did previous seasons, there's a new mini-Baron called the Rift Herald that will allow 1 person on your team to push faster, teams with a level disadvantage will have minions (on average) closer to their turrets through a mini buff, and death timers become more critical earlier into games.
Vision got reworked so that there needs to be more effort put into vision this season to get the same results as before. Stealth wards can no longer be purchased, only vision wards can for 75 gold. Sightstone can now build into 4 items with the only one worthwhile pre-full build being the Ruby Sightstone. Yellow trinket drops wards whose lifespan scales with your level being shorter than the lifespan of previous wards. That trinket can be upgraded to the blue trinket at level 9 which drops a visible ward with 1 health and no time limit at its cast location on top of revealing the area for a few seconds. Red trinket functions the same as before and you can upgrade that at level 9 for one that follows your champ as you move. Red trinket also creates a shadow of stealthed enemy champs when they are scanned to reveal their approximate location.
New items were added such as Death's Dance and Deadman's Plate, items were reworked like Phatom Dancer and Essence Reaver, and items were removed/swapped such as Twin Shadows being removed and Brutalizer being shattered into Caulfield's Warhammer and Serrated Dirk.
The mastery system got redone. Keystone Masteries is now a term used to describe the maximum mastery in a tree.
META
Many mid laners are taking the current Frost Queen's Claim item very early because its active is incredibly strong. That item also appears to be the strongest item for supports at the moment.
Dragons and Baron are just as important as ever but nobody seems to be taking the Rift Herald because of the multi-man investment that usually requires 1 person to recall after killing it.
The Keystone Mastery called "Thunderlord's Decree" is currently the most favored mastery with over 5 champs per game on average taking it as any role.
Balance... it's like dogs and cats living together, complete hysteria. Not a frequent series of tweaks, but there's a good number of champs being complained about at any given time this preseason. Like more than usual.
If you run Sated Devourer, you may want to consider the new Guinsoo's Rageblade since its buffs.
Grievous Wounds is now an appealing option for some AD champions, but nobody buys it because... NA is best server.
NEWS
The next champ to be released nicknamed "Jhin the Deadeye" appears to be coming to the PBE soon and has had numerous teasers.
The next class of champs to receive reworks will be the immobile mages, specific champion names not yet provided. It is expected to happen around June.
The next large champion reworks appear to be Shen then Taric. No estimated dates on their releases.
BRONZE TIPS
Usually people in bronze have a number of things they need to work on in order to go up in the ranks, some basic while some may take some time.
First, have your mastery pages setup before you enter a queue. Make sure that all of your pages have all 30 points spent.
Secondly, make sure you have a solid rune setup. I'm not talking about looking up what the pros use, just make sure that all of your runes are greater runes, and that you split a given rune type across no more than 2 stats. Ideally have at least 2 unique full rune pages setup.
If you're a lane carry, focus on farming. Farming 3 waves of minions will grant (on average) more gold than a champion kill.
If you're a jungler, assess the situation across the map and adjust accordingly. Just because you're a Sated Devourer jungler doesn't mean you NEED to avoid ganking lanes. Also make sure to secure Dragon when you can do so safely and understand that burning an enemy summoner spell or ultimate means the gank was a success.
If you're a support, understand zones of safety. If the enemy team has a scary Blitzcrank, abuse his 20 second cooldown early on as a free period to harass both opponents. An enemy marksman is often vulnerable when they go to hit a minion, then would be a good time to try and hit them. Keep a level head during hectic moments and use exhaust/ignite on threats, ie) if anyone else is around, not the enemy support.
Also, warding is everyone's job. Know what trinket you want to use and use it to its fullest. Everyone is given a trinket at the start of the game and it should be used. Early game its cooldown may incentivize you use it if you suspect something may happen around a bush, but mid-late game feel free to use it whenever it's up. Even if you're not able to ward anything too important, lighting up a random dark bush does your entire team more help than keeping it in your inventory.
Make sure to remain dynamic in your approach to builds. If you play a champ top lane that normally rushes Spirit Visage but you're against an AD laner, consider getting armor before magic resist since the magic resist will be much more patient than your lane opponent. If everyone on the enemy team has magic resist, magic penetration will be helpful against everyone you hit (if you deal magic damage). If the enemy team's AD champs are getting much stronger than their AP champs, prioritize armor over magic resist. If you normally buy one damage item then go tank, but fall behind early, don't feel obligated to finish your damage item before building defensively.
Mastering a small pool of champs is better than merely having a hands-on understanding how 40 of them work. It's quite often that the higher you climb in ranks the more "role specialists" you'll run into. That being said even if it's only 1 champ, do learn champs that play in other positions than your main. I insist on visiting some of your favorite champs' wiki pages to see what intricacies you can find on their spells and strategies.
Lastly, miracles happen. Pro games have ended with the enemy Nexus being 1 auto away from being destroyed, they got aced, then the other team won. In bronze, your opponents are far more likely to throw than the pros. 4v5 is still winnable, never go AFK in a match, and if you're tilting then take a break and come back once you can think clearly again. A game can still be won as long as your Nexus has at least 1 HP left.