If you're
and you have a lead, try to get a couple of extra kills on the enemy mid laner if you can, and push to their tower. If they're playing super-defensively, that's your cue to roam to another lane and snowball it. Shove your lane to the enemy tower, look for a suitable lane to gank (i.e. your target's close to or past the midpoint in their lane) and roam there. If your entire team's pushing to their opponents' tower, just continue shoving your own lane until an opportunity comes up. In the worst case, you'll get an early tower, which will help your allies a lot after the laning phase ends.
Some general advice for every game, regardless of whether you're winning or losing, is to stay positive and be friendly to your allies. If you don't have anything positive to say, say nothing. If you see one of your allies is having trouble, try helping them if you can, and otherwise try to make the most of your current situation. If one or more members of your team are being toxic, just mute them.