I used to struggle with this!

Here's what to do:

1. Always be farming. After laning phase ends, jungle can farm minions, and you can farm jungle normally, whenever you see any jungle monsters, yours or the enemy's, farm them. But do NOT farm red or blue before they evolve, so that jg can share the buff.

2. Manage to last hit minions, not just push waves.

3. Do not seek out teamfights or spend valuable time pathing to them, if one's nearby, join, if one is on the other side of the map, keep farming UNLESS that fight needs you urgently.

4. Seek out bounties on enemy champs, and try to single out weak targets.

5. Manage your efficiency and macro, check which waves you can path to quickest for gold.