Master yi - to be great, it takes some skill with q-timing so you can dodge enemy abilities, but even without this, he's still good.
Xin - easy ganks, good at pretty much all points of the game. Some nice sustain even without lifesteal. You do need early tiamat if you want to farm raptors and krugs, but otherwise his jungle clear is easy with just starting item.
*Do not build full damage on these champs. A) unnecessary B) leaves you very squishy, so engaging at the wrong time is more easily punished C) you probably have to be a frontliner for your team. I recommend 1-2 damage items into 1-2 resistance items, then a final damage item. Depends on if enemy team is fed and has burst damage. Like if there's a fed malz or annie, building an mr+hp item after bloodrazor is way better than building bortk second item - because annie will kill you before you can make use of the lifesteal.
Warwick - similar to xin. Builds tankier, but still decent damage. Also multiple forms of cc which is great.
Cho'gath... no explanation needed other than - start e. Raptors is perfect first camp with him and the best first camp to do if possible for a jungler.
Gragas - takes a few games to learn your damage and how to ult to split up enemy team, but that's it. Otherwise pretty straightforward and solid pick.
Also - know what the purpose of jungle is. Getting laners ahead without costing your own relevance - not hard farming and trying to solo carry. The xp just isn't set up for that right now, plus it's low elo - leaving teammates to fend for themselves for 20 min means they are likely to feed and quit or not try their best. Even if you are a jungler who wants level 6 before you gank (noc, ww, sej), try to gank at least once or twice before lvl 6. Don't force stuff that's not there, but if an enemy is pushed, even getting some nice chunk damage on them to give your laner an advantage in hp is appreciated and useful.
If you only have 1 tank/frontliner, it's especially important that the enemy DOES NOT get to camp them and put them behind. If you allow this, you have 0 protection for your carries - so make sure to gank/counter-gank for them early and often as needed to ensure they have a good game. Look at cs, lvls, and stuff too - not just kda when deciding where to gank. A lane that has been camped for 10 straight minutes and is 0/2/0, but ahead in cs and even in levels is actually not a horrible lane to counter-gank for. The whole of the stats shows that it literally takes a 2v1 to kill them - meaning you making it a 2v2 probably gets you an even trade or better - and your laner will begin to pull ahead since they are farming better and have equal levels even before getting help from you. On the flip side - if a lane is feeding horribly, down significantly in level/xp and cs - do not gank for them regardless of if they were feeding 2v2, 1v1, 1v2, etc. This will only put you behind and give you unfavorable trades - look to punish by ganking other lanes.
Consider your laners. Gank when the opportunity presents itself. If an enemy and your teammate are both low and you are nearby - gank NOW - not after doing one more camp. Waiting means your ally probably dies - either 1v1, or to an enemy jungler beating you to the punch. Or best case scenario - the enemy laner just recalls and you miss an opportunity. If your ally has a wave pushing into them - don't go and take cs while they are basing or dead. If your teammate can make it top in time to catch the wave at or just outside their tower, it will help them a ton. If it's pushing to their opponent and you CAN SAFELY do so - shove it to tower and reset the wave for them. Considering when to leave their cs alone and when to push it in can mean lane freezes to help them (and set up easy ganks for you), denying such freezes by their opponents, or if you screw it up - the opposite. There's multiple paths into enemy lanes to gank them. Too many junglers only try ganks through the river - even when they are pinged it is warded. Lane gank through friendly tower, through enemy jungle, through enemy tower are also options. Finally in this regards - listen to your laners. If they ping you NOT to gank, there's a reason for it. Like maybe they realize you can't win the 2v1 or they know the enemy jungler is already hiding on your path to lane, or they just used all their abilities and can't engage for a few seconds, or there's a giant minion wave they need to clear so you don't die to minions in an attempt to kill the enemy laner. It doesn't mean they never want you to gank - just not right now.
Know your matchups going into the game. Will the enemy jungler look to farm early, invade your jungle, or gank lanes a lot? How will they do this (strategies)? What about each of your lanes - do they win/lose/go even? Do they push? Are they easily ganked if they don't have backup? Same for enemy lanes. Where can you most easily combo your abilities with those of a teammate to look for kills. Where did the enemy jungler probably start - look to what enemies are late to lane. I know it's a lot of stuff, but keeping even some of these in mind and making them habit will help you be a great jungler and not just a guy/girl filling the jungle role. This isn't a complete list of stuff to keep track of, but it's a good start.