Jungling is one of the more complicated roles in the game and there are a lot of things to take into account at any given time. Ganking and farming come with tradeoffs. If you choose to gank you will lose out on some farm and the enemy will see your location (a big deal for high level jungling) but can force an opponent out of lane, miss farm, or secure a kill/assist. You need to pay attention to who you are ganking, where they are when they will first see you and be able to react, relative strengths at that moment and many other things.
How often to gank will also vary. Lots of successful ganks can help you get some of your lanes fed and might feed you as well. Unsuccessful ganks though can result in you falling behind enemy junglers if they continue to farm, this is true even if you don't die simply because you are getting no exp and no farm while running to and from ganks.
Counter-jungling likewise will depend on your opponent's jungler and you. Certain champions are relatively easy to counter-jungle with and against while others are more difficult. In general though awareness of your current strength compared to the enemy jungler will help (if they show at top and you are at bottom it will be quite a while before they can even get to bottom to potentially cause you problems while in their jungle) Also be aware of your team and the opponent positions. If you get into a fight while counter-jungling you will probably bring the other opponents to you. In this respect a 1-v-1 you are winning can quickly turn into a 2-v-1 or a 3-v-1 if you are not careful and quick.
Specifics are hard to discuss without a whole essay but maybe these insights will be helpful in your decision making. Because that is really the key to successful jungling is proper decision making.
