If you want to back to get items, you should push your minion wave as far out as possible so your opponent would have to spend some time wave-clearing before they get to your turret. In some circumstances, it's good to let your opponent push and just farm under tower (such as against Fizz) and in others, it's better to stay passive and let the lane go either way. Don't overextend unless you have wards in the river or if your jungler is not nearby to countergank.
It's good to get early harass but DO NOT trade autos level 1. You may or may not have noticed that whenever you attack your enemy near creeps, they will focus you and you will take minion aggro. Minion aggro level 1 is very high so it's best to just last hit until level 2 or 3, when you can harass with spells or autos. Generally, you don't want to harass when there's a big enemy wave building up because the amount of aggro you receive will increase. If the enemy, however chooses to play aggressively and constantly pokes you, you NEED to follow up with damage to them, or else you basically let them put out free damage, which is not what you want. Observe the enemy and if their playstyle suddenly changes (ex: they play passive throughout the entire laning phase and suddenly moves toward you aggressively) it usually means there's a gank coming. Be wary and frequently adapt and have good map awareness. Also, pay attention to when they use ult or summoners because if you have the ignite advantage, it can really open up a kill for you. Knowing when to go all-in for a kill is really important, and it varies depending on the champion you're up against. I main
and if I'm laning against, say, a
I will try to passively poke him out until he uses his jump and then go a bit more all-in, as he has less defense now. Or as another example, if I'm against
I will wait for her to use Charm, dodge it, and then go in to harass. A good time to go for kills is also when the enemy is low on mana, which is another reason why last hitting and harassing often with autos and not spamming your spells is so important.
Is broken af, but every one I've played against used his Q continuously to farm and try to harass. Only thing is, it's easily dodged and he'll end up super low on mana while I have a full hp and mana bar.
Also, one more thing: you don't win games just by having kills. You ultimately win a game by killing the enemy nexus and to do that, you need turrets. Meaning, every time you secure a kill, make sure you get an objective with it as well. I see so many people backing after a kill and then end up losing their turret. After you kill the enemy midlaner, you push your wave and try to get as many hits at the turret as possible, and get out before the jungler comes and kill you. The only thing to keep in mind is to not attack the turret before 7:00, as you do reduced damage. Just let the creeps do the hitting and focus on killing enemy minions that approach.