What do do as jungler?
The most common approach is to first clear the midcamp on the side of the solo lane together with the mid laner and the solo laner. After that the mid laner goes to his lane and you do two jungle buffs with the support. Depending on your god you either take damage or speed. Then you take the mana buff with the solo laner.
Throughout the game make sure you get to your jungle camps in time, gank mid and solo lane as much as you can to get a experience advantage.
Making a jungle camp in time means that you are there when it spawns. Later in the game this isn't as much of a priority anymore since you spend your time ganking lanes.
Its also your task to call for a Gold Fury or Fire Giant attempt.
Another popular approach is invading. The mid laner takes the midcamp on his own while you and the support try to take the opponents mana buff. They are likely to be here with their jungle, if you can try to steal their mana camp. You don't even have to obtain it since they can't pick it up anymore. This sets back the enemies solo laner a lot since he constantly needs to get back to base for mana.
I am sure there are a lot of other approaches but these are the most common ones.
Your build is important too, some people start with a 800 gold item and a damage potion which is 400 gold. This is a risk because if you die you lose 400 gold and get behind on the enemies jungler!
Keep in mind that clearing jungle camps is a high early game priority. So you may want to buy some items that help with that, for example 'Hand of the Gods' this can later also be used to kill the Gold Fury or Fire Giant when they are below 25% (Only if its build out).
If someone dies in a team fight and the enemy can push because there is nobody who can defend it you have to be a stand in for this lane and defend it until someone gets back to lane.