some good goals are 80 cs at 10 minutes. after that teams start to group up and your cs can suffer quite a bit, so its not always a good idea to have a cs goal for later in the game.
understanding the mechanics of every champ in the game, or at least the ones that see play. doesnt mean you have to know them in and out, just know what their moves do. this will help you with knowing what your team is capable of and the enemy team
its good to have at least 2 champs you can play in every role so its harder to get counterpicked/stuck in a role you can't play
be willing to play any role thats available, and be at least decent at the role. ive not found anything more helpful to my team than just letting them know they can grab whatever role they feel best with and ill be able to pick up whats left without hurting my team
it also helps if your high on the pick list (1st or 2nd) to pick a role thats harder to counter ex. jungle/support/adc- these roles arent countered as hard as top and mid. some people may disagree with the adc pick first but since its a duo lane if the other champion tries to counterpick bottom the support should still have a pick left to try and counter their pick, or at least even out the advantage a little bit
buy wards-even as an adc (within reason of course). there is no person on the team that should never buy wards. vision=gold. it either grants a kill or prevents a kill. makes it easier to contest objectives like dragon and baron
NEVER BE DOING NOTHING-possibly the most important rule in league. always be doing something. buying, farming, pushing towers, getting buffs, killing jungle creep, ganking other lanes, contesting/getting dragon/baron, warding/counterwarding. basically if you're wandering around with no clue what to do-you're doing something terribly wrong and need to watch a replay maybe to see what else you could be doing
split pushing is super effective. especially in low elo-alot of people cant react properly, or execute a split push properly-make sure your team KNOWS that you are split pushing. don't want them to get caught in a 5v4-they'll just blame you. also, you have to know how to split push. this means ward your route and have an escape plan ready. shaco, shen, and tf are an example of excellent split pushers. they have the ability to push down minion waves quickly, and a safe escape mechanism (shen utli, tf ulti, shaco deceive+JitB)
when in champ select try to pick a champion than benefits your teams composition, this will come with practice only. once you understand other champions better you will know what they excel best at. for example shen is great at split pushin, vlad ulti is great for aoe comps, maokai ulti is great against aoe teams, cait is great at poking from a distance, ect. also remember to keep an eye out for the following things-does your team have a tank/initiator (you will need at least 1 beefy person to soak up the burst damage)? does your team have any cc?(a team with little to know cc is likely to lose against a team that has reliable/effective cc).
suggestions may be a bit scattered (currently at work and keep getting interrupted to actually do my job) but hopefully these little tips will help!-oh and one last one, always be friendly. makes the angry people even more upset (trololololol) and the people that are actually trying try harder and not feel so bad about it, after all-its still just a game