After youve leveled up a bit, and maybe even took down a tower (or lost one), people will start to roam around the map trying to knock down towers in different lanes, trying to kill enemies, or trying to place wards to give vision. This is known as mid game. No one really announces when to start doing this, but you will develop a feel for when its time to move out of your lane (or when your teammate is screaming at you).
Im like level 10 now, where do I go?: This depends on whats happening in your game. Maybe you need to go to another lane to defend or attack a tower. Maybe an enemy is in an open spot where you can kill them. Maybe you need to put more wards down. Maybe you need to stay right next to your tower because the entire enemy team is coming for you. Talk to your team and assess whats happening.