There are literally thousands of things one can do to improve and get better at the game. Start by setting reasonable goals for your self to improve upon.
Trying to all ways get a minimum of 100cs by 13 minutes. Just being able to CS can help you win more games, because more cs=more gold. You don't all ways need to kill your lane opponent in order to win.
Learn to ward and when the best time it is to ward. If you just saw the enemy jungler and mid laner in the top lane it doesn't make a whole lot of sense to waste a trinket ward on the bot river bush at that moment in time, but I see people do this and by time the enemy jungler makes it to bot for the gank their ward is gone and they need to back or they died.
Have a good champion pool so you can play all roles in the game decently enough so when your not in your main role you can still contribute to the over all game play. This can also be a double-edged sword. I've seen people have a nice champion pool but they play so many champions they haven't mastered any.
If your losing in your lane don't just sit in lane and be useless not collecting any cs, especially if the enemy laner has it froze or is zoning you hard. Go roam or ward the enemy jungle do something don't just waste time.
Learn how to properly build and when to pick up certain items and when not to. For example, let say your the ADC and your constatnly getting caught out and dying to lux. Lux is all ways landing that damn Q on you. Building more damage isn't going to ave you from that combo but getting a
or
can save you from that deadly snare.
Look at the mini map every 10-20 seconds minimum. If someone doesn't call an MIA then it is up to you to look at the mini map and see who is missing.
There is so much more I could put here but I'll leave it as is for now. If you have any more questions feel free to ask.
The only other point I want to add is Never blame other peopel for your loss. All ways take every loss as your fault and ask your self what you could have done better? There is all ways something to improve on even when you don't think there is. That is also why recording your games can help a lot to.