What do I do in mid and late game.

depends on the champ you play as. list me some and ill tell ya.

as Radnar says...it depends on your champion. For the most part...you want to do some rotating early if you get your tower down...if your tower is down and your lane opponent is gone...start moving down mid lane to help gank/push.

Generally as a top laner...you're going to be alone...a LOT. At least if you're running teleport. You want to split push your lane and maintain pressure while keeping in constant conversation with your teammates on when and where to teleport into a fight. If you're using a homeguard champ like Maokai or Hecarim...you're going to want to recall before your team makes their move and then immediately teleport behind the team onto a ward as either initiation or to cutoff the retreat.

If you're not running teleport...you're going to want to mostly pack run with your team after the outer turrets are down (first tier turrets) as you're going to want to be present for team fights and you'll have to rotate as a team from mid to top or mid to bot once team fights are one to push objectives as a team.

I play riven mostly unless it seems like a really bad pick