I find that when I want to improve, I actively pick one thing to work on until I get better at it. And there are lots of things to this game.
Some basic things to get better at :
Die less - when you die, you are not just giving the enemy team 450g, you are giving them time without you on the map, which means they are getting even further ahead. Aim for < 5. (I avg ~ 3)
CS more - If you are a laner, you should be shooting for 5-6/m minimum.
Ward - Make sure you are placing your yellows, upgrade at 9 (most don't at lower tiers).
Keep an eye on the mini - This goes hand in hand with warding. If you are warding, but not checking the mini to see what the wards see, then.....
Use pings. MIA is probably the most important.
Don't chase, dive, or otherwise make risky plays if you aren't 75% sure of the outcome or have vision. SO often, I see people with 2-3 bars of health reengage a retreating enemy that is at more than half health...then they wonder why they die 2 seconds later.
An addendum to the above....RECALL if you are low. So many people stick around to get a few extra gold to buy that item or think they can get that enemy who is alos low. DON'T. I see this backfire more often than not....even a 50% chance isn't good.
Shop fast - If you die, shop when you are dead. If you are backing, you shouldn't spend more than 2-3s buying new stuff and running back out. I see so many people back before they have enough gold for something (so they wait) or they are looking around the items (every second you spend at spawn is a second the enemy is getting ahead). (Use item sets.)
The list goes on and on....but these. I think, are a good start.
