Starting playing league of legends

Make sure to try out every lane and experience it by yourself (same for champions).

Every lane has it's purpose just like every champion.

There are assassins, tanks, fighters, long range AD carries and supporters.

Once you found out what you enjoy the most, you can start improving on that role and learn more champions in that role.
Also do not watch streamers playing untill you understand the basics of the game. Watching it will make it worse because you won't have a clue why he is walking to a certain point or why he thinks fighting there is a good idea etc.

Most of the experience comes with time but even more important is to make sure to NOT make the same mistakes you did in a bad game. Tho that doesnt mean that a lost game is necessarily a bad game.

Take every mistake that you notice and keep it in your mind when you go into the next game. You will remind yourself when you are about to make the same mistake and with time it will happen automatically that you avoid "bad" situations, ward when necessary or even when to pick a certain champion (once you start playing ranked.). If you play the game to "win" only you won't have fun for a long time. Play the game when you feel like it.

I hope this somewhat helped you understand what I meant. Sorry for the bad english :/
GL and HF in your future games.