Zac is prob the easiest jng to use if you are learning the basics.
You start at blue, hit gromp, hit wolves, get your red, and gank, gank, gank. need health? river scuttler. Look for that lvl 3 mid gank as soon as you kill your red.

Mid game is all about objectives. Do your best to help your top or bot lane win lane. Zac cannot carry. So don't gank severely losing lanes. just gank another lane instead. If your team is pushing their waves, let them know you will dive (post-lvl 6 - pref). dive in with a minion wave and R, Q them to let your teammate have the kill. EZ win.

Late game will be largely teamfights. Try to engage teamfights from the jungle diving into the lane. Made sure that after you engage you IMMEDIATELY start to peel for your carries (ADC, MID). Peel the enemy off of them like a banana. You zone and peel and you will win.