Last Hitting for new players!
I feel that one of the most important thing for new comers is to practice on their last hitting! Whether it would be on minions or players, always practice on last hitting, especially the minions so you can farm more efficiently. The way you can do this is to click around and once you see a minion down to it's last bit of health, hit it to get the farm.
And for your opponent, always try to out-trade them. By that, I do not mean to get greedy and die while trying to out trade, but try and hit them, walk away, and repeat (kiting). If you feel that they will do much more damage to you then you'll do to them, do not try and engage, just stick to last hitting minions (farming).
Hope I helped :)