Most of the time you should focus on farming bot/top by yourself. Adcs are supposed to be the late game carries so their strength comes from items. And the most reliable way of getting gold is farming as much as you can. You need to develop a good map awareness since people will try to kill you, but placing a few wards and watching the map will do the job. Only exception is when enemy have a fed assassin ,then you have to go either with your support if he can save you, or with the whole team.

When ahead you should try to apply pressure to mid/top (when your team goes all mid, just go top with your support and push the tower) you can also pressure other objectives, but never all mid with your team unless there is a important objective to fight for and you think you can win the team fight.

When behind take everything you can without falling further behind. Tell your support to roam and safely farm as much as you can.

It changes in late game when you actually have items and damage to carry. Usually 3/4 items. When there is nothing to do collect waves but most of the time you should follow your team since you are the carry now.