There's no hard and fast rule. You can pick the person on your team who you think has the best chance of carrying you to a win and you work on feeding them. It can be a nasus top, it can be your adc. It can be an uncommonly good mid Zilean. It can be yourself, depending on your lanes and your choice of a jungler. Try to find the person on your team doing the best and/or playing a champion who can really help out come mid to lategame teamfights, and try to feed them.
Sometimes you'll wanna gank a lane JUST to shut down the enemy in it to make sure THEY can't carry. Sometimes you'll need to avoid a lane entirely because it's beyond saving... but that's very, very bad. If a lane is too weak for you to turn the tides, they're gonna shove it hard and take things. Often, this is bot lane. Often, this gives the enemy dragon control. Try to coordinate a two-man gank with another lane. Maybe mid can roam top or bot to help out. Maybe bot can come mid. If you give up on a failing lane, the odds of them coming back are slimmer so always look for some way to help, even if it's "camp top to draw the jungler to you and get pressure off of bot".
Jungle is, I think, the hardest role in the game because of all the choices you have and how much they affect the game as a whole. You can influence a LOT from the jungle, and you have a lot of decisions to make on incomplete information. Do you gank top? Mid? Bot? Just farm? Is their jungler ganking? Do you try to help or is it a lost cause? Can you invade? Can THEY invade? Gotta keep your buffs safe from counter-jungling. Should you try for Dragon? Baron? Where is their jungler? Can you sneak some wards into the enemy jungle? Did the ward River? It goes on and on... and it's a lot of details to track and plans to worry about. Bad or mediocre junglers free farm, gank if they feel like for who they feel like, and maybe keep an eye on Dragon. GOOD junglers ward, counterjungle, snowball lanes, prevent ganks, pressure lanes, RELIEVE pressure on lanes, counter-gank, rally their team for objectives and know who to feed (or prevent from getting fed) and at least 3 different ways to get to them despite the river being warded. It takes a LOT of time, experience and practice.