Gank your lanes.
In all seriousness, if you're going to be the jungler it's your job to make sure that your team does not get ganked by the other team. It's your job to watch the map, since you don't have to worry about last hitting the jungle creeps, while laners have to focus on last hitting minions, dodging the other champ's skillshots, to watch the map much. Always help your team ping if you see a missing champion from lane, and aways ping if you think a lane is going to ganked. Try to provide vision and jungle near lanes that are pushing too far ahead, so you can get to them faster if they get ganked. Do not gank a lane if the laner's health is really low and their opponent's health is still really high, you're just going to get them killed if you gank that lane. If you think the opponent might dive them, camp near by, wait for them to turret dive, and THEN gank them.