I'd suggest Warwick. I was very frightened about learning to jungle (I had some very poor experiences when I was experimenting with it). PsiGuard's Warwick Guide was really all I needed to get to a point of comfort.
But these days, sometimes Warwick gets banned in draft and ranked. I'd suggest learning a second one also if you like to play Ranked or Draft.
Master Yi is not very hard to pick up and is rewarding when you use him right. He has sustain with his Meditate (W), extra Damage with his Wuju Style (E) and also can dodge damage with his Alpha Strike (Q). Even tho his ganks can be a bit weak in early, with the slow from a Red Buff he can be a real danger even early on. Max your Q, buy a Bloodrazor and from there just go with one of the fine builds on this site.
Maokai is a relatively strong, and simple jungler for beginners. He is extremely tanky, and has many forms of cc (crowd control) to help you get out if you get caught in a sticky situation
But these days, sometimes Warwick gets banned in draft and ranked. I'd suggest learning a second one also if you like to play Ranked or Draft.
Hope this helps,
~moon