I'll give you some advice as a jungle main. First, run through your first clear several times in custom games until you have it down as efficiently as possible. The first three camps are almost entirely invariable in when you get them, what skills you use, etc., so optimizing this will be a good boost to the speed with which you can do your first clear, as well as giving you a general idea of what your health pool will look like after you finish your third camp. For Rengar I would highly recommend starting krugs, and then moving to red buff, and then either wolves or blue. The passive you get from smiting krugs saves you a ton of health in your first clear, because of your auto-reset with your q.
Next up, if you start red side, you should do krugs first for the smite bonus, followed by red, and then a camp of your choosing. If you start blue side, do gromp first for the smite bonus, do blue, and then whichever camp you like. Also, when you start your first camp, smite immediately so you get the maximum effectiveness out of the smite bonus.
As far as ganking goes, it varys a lot by champ, but with Rengar you should wait until 6, and then gank whenever your ult is off cooldown. If it is on cooldown you should be farming up in the jungle. A big problem when you start out is you will find it difficult to manage your time well, and will end up wasting a lot of time not being sure what exactly to do. So just make sure you are always doing something, and going at it all the way. Half-assing a gank because "they told me to come", is far worse than ignoring the lane and just farming your jungle. Which brings me to my next point:
Laners have no idea when is a good time for you to gank (usually). You will often feel pressured to gank for lanes because laners are pinging and asking for help. Remember that despite this, it is your decision what to be doing with your time. If you think that top lane would be a better gank target than mid, despite mid pinging like crazy and top being silent, then go for it. If you think that the enemy leblanc has too many escapes and isn't worth ganking, feel free to farm, even though your mid is crying about wanting a gank. Remember, you're the jungler, and it is entirely up to you what that means.
As far as ganking goes, there are a couple of things to keep in mind. Firstly, look to see if the lane has a lot of escapes (flash and abilities). If they have escape abilities (ez, leblanc, etc.), wait until they use their escape move (since they will usually use it aggressively if they are not anticipating a gank), and then immediately go in once they have used it. Same thing if the lane has a lot of disengage (alistar, janna, etc.). Secondly, if you have a gap closer, it is best to wait to use it until after the opposing laner has used their escape. On Xin for instance, this would mean holding your e until the opposing laner flashes, dashes, etc.. Rengar is a bit special because of his ult, as you should be initiating with it, but as a general rule of thumb, it is good to hold onto gap closers. Lastly, check to see how your laner can contribute to your gank. If you're ganking for a lane with a lot of cc, you will have a much easier time than if you are ganking for a lane with little or no cc.
Now I'll get to your specific questions. Your first question is about when to grab blue if you don't get it on your first clear. I would say that you should get it asap as you want it back up asap. It's free gold, and you can start donating your blue to your laner sooner if you take it out earlier. Secondly, you ask about whether ganking or farming is more important. For this you need to consider who you are playing. If you are playing someone who is very strong early, but weak late (read lee sin), than you should focus on ganking almost entirely. If you are someone who is super strong late but weak early (or have to reach a certain threshold in order to become strong), then you should focus on farming early until you hit the desired threshold, and then focus on ganking (warwick, rengar, amumu, etc.). If you are playing someone who is exceptionally weak early, but who gets very strong late game, you should focus almost entirely on farming, taking only the most opportunistic of ganks (Mundo, Shyvanna, Master Yi, etc.)
Next, you ask about how often you should gank once you hit six, to which the answer is, every time your ult is up. Rengar is pretty pathetic with ganks unless he either A) has his ult, or B) performs an in-lane gank (walking into the bushes on the side). If you can't meet one of these two conditions, focus on farming only (though some counter-jungling would also be a pretty good idea considering rengar's assassination potential.) Regarding your last question, you should start grouping up whenever your team is doing so. If your team is group, join them. If your team is chilling in their lanes, just chill in the jungle. Also, when you reach mid-late game, and everyone is moving everywhere, try to weave in a camp or two as you move around, but don't worry about farming the jungle too much. The jungle should become a free gold/exp deposit for your team at that point to be farmed by any member of the team who is in the general vicinity.
Finally, feel free to ask me any more questions you might have. I've been playing since season 1 and have mained jungle since mid-season 3, so I'm fairly knowledgable about such things.
+ health pots and a ward; you need the purple smite for the sustain. Then clear the other side, looking for opportunities to gank. The key word here is OPPORTUNITY; it's hard to force a pre-6 gank with Rengar, so you're most effective at counter-ganking (going in while the other jungler is trying to gank) or punishing an enemy for over-extending (gank while they're fully committing to a lane kill). Look for opportunities, but you're priority is to farm up items and get to level 6.
(sustain, AD, burst; you can activate the active while you're leaping towards someone)
(active gives an extremely useful movement speed boost during ult, gives other useful stats)
(once you get this as your 4th or 5th item you are basically the angel of death)
and/or 
work pretty well. I haven't tried it yet, but the movespeed + decent stats mean that 
is probably solid for him too.
(if you fall behind and find yourself having to duel tanks more than you'd like, this isn't a terrible choice)
(if you're falling behind to an AP heavy team, this is a solid early game pickup)
(pretty justifiable purchase; gives tankiness, all-important CDR, plus armor shred that helps your AD teammates a lot)
a try. He's great at clearing the jungle, has simple but satisfying mechanics, is super-strong early game with amazing ganks, and is the same kind of carry jungler Rengar is (just not an assassin).
or 
(for ADC) i'd be the richest LoL player alive.
