Well, the fact alone that you know you're not the best and want to improve is a great big step in the right direction. I'm not really great myself, but I can give basic tips... although they're the ones you hear the most.
- Ward. Always. Upgrade your trinket as fast as possible, ESPECIALLY if you're getting
- Don't rage. Don't play while you're salty. Don't yell at teammates. Your team doesn't need any of those, and what your team doesn't need, you don't need.
- Counterpicks don't win games. Don't play ranked with a champion you can't play. Sure, counterpicks definitely help, but they don't decide anything. Also, when counterpicking, make sure you know why champion A counters champion B, and abuse that.
Example:
counters 
. Not because Lux can insta-burst Veigar, but because she outranges him. A good Lux keeps her distance, poking him down and executing with Final Spark. A bad one goes close to hit easier, then gets instantly killed.
- Try to learn how other roles than your typical ones work. Knowing how an assassin operates helps the Tank or Support protect the carries from them, since they would know how an assassin might approach the situation.
- Communicate! This one is important. Don't talk too much, but try not to communicate with pings only. If someone does well, tell them. If doing baron is a bad idea, tell them. However! If you're gonna tell your ally how bad they are... DON'T. Just don't. Again, refering to point 2: "What your team doesn't need, you don't need."
- This one is just as important as 5: LISTEN. Your team may, in quite a few cases, know better. Be open and listen to them. Teamwork isn't telling one guy how to do things, then watch them try and fail, then yell at them for doing it wrong. It's speaking, listening, and helping eachother do it right.
Again, these are some of the most common tips, but they help. If you already know them, then sorry, I can't help unless there is something specific you need tips on. But good luck getting better! I hope it goes well.
