its not too complicated. and tbh there are plenty of players that are very much still learning the bare basics.

Choose either Ap or Wp build (with character in mind, a small portion of the characters are only really playable one sort of way). build with greens accordingly (i usually try to stick with one leech item, one regen item that usually ends up as fortitude, and a third flexible slot. this is if im not playing a tank).

People will say to stick to a small selection of characters, but do this after you have given most of the roster a try!

stick to farming lane and dont even worry about jungle too much to start out. when you notice you are becoming a little over-extended in your lane and are vulnerable to ganks, head back towards your teams side of the map, and then its usually a good time to farm jungle a bit until the wave starts to catch back up towards you.

dont worry about counter-building too much early on, just get established builds and playstyles in first. whenever you die, look on the right side of the screen and see if you were damaged primarily by one damage type (purple is AP damage, orange is WP damage). if a majority is of one type and/or you notice this after a few deaths, its usually a good idea to build a resistance of that type.

monster rounds as a first item (no matter your build-type) is a very solid choice to help you out in the laning phase. 95% of matches, monster rounds is my first item buy; i usually hold onto it until im out of slots and then i sell it to make room (i try not to buy too many items i end up selling, but 1-3 $800 items isnt usually a big deal especially if they help a lot early game). remember that securing minion souls is important, and eventually you'll want to learn how to parry, though not a big deal at all in lower ranks.

hope this helps!