It's not easy to determine where/when to gank, it's something you mostly gain through experience. That being said, I'll try to show you factors you should consider when deciding whether to gank a lane.
Likelihood of success:
I'll start with this one because you seem to already have it down. Be wary of your champion's ganking power and tower-diving prowess relative to the enemy's survivability. Watch out for warded and extended lanes -- most top laners ward their bush at 2:30. Keep track of which lanes have
on cooldown, those are easy to gank. Be also wary of which champions are easy to gank (mages, bruisers, immobile bot lanes) and which champions are hard to gank (Lulu/Janna, mobile ADCs, Illaoi, low-HP teammates, assassins). If my bot lane has low-CC and they're against Janna/Xayah, I literally never even bother showing up bot unless they're under tower.
Objectives:
When you gank, you must that it costs you pressure on the other side of the map. Even if top is an easy gank, you should be aware of what you'd lose by going there. If Dragon is up at 8+ mins, you probably shouldn't show your face top. There are many factors to this. though. Farmers

are a lot more likely to try and take Dragon than gankers
. Additionally, if a lane is fed, you can probably trust them not to lose a tower or Dragon by going elsewhere, but if the enemy is fed, you might wanna stay nearby if you feel like you'll lose that tower or Dragon.
Buffs:
Similarly, if the enemy jungler has been applying pressure at your buffs, you probably shouldn't show your face at the opposite side of the map unless you're willing to lose that buff. It's a calculated risk. Is a chance at bot kills worth losing red buff? Do you need red buff to reach 6 in the first place? What if you can gank bot AND trade red buffs with the enemy jungler?
The enemy jungler:
There are various jungle paths in the early game. Some champs are slow, some are fast, some take scuttle. You should be aware of what path the enemy jungler is using, and how quickly they'll clear it.
might start red then go for a level 2 gank.
might simply do buff->wolves->buff then try to gank top or mid.
might try a raptors->krugs->wolves->gromp->blue->back->red->repeat path to reach level 6 ASAP. If you don't know the path, try to drop a ward in the enemy jungle to see if they're going where you think they are. Warding buffs is always nice if you can reach there. I also like to ward raptors.
Experience and playing various junglers will teach you which path a champion likes to do and how quickly they can clear it. Being able to anticipate that pathing is huge, and lets you carry out deadly counter-ganks. Or jungle invades. Or take the enemy's opposite-side jungle. Or gank the opposite side of the map. Don't think you and your top can 2v2 the enemy top and jungle? Run bot and fuck them up while you know the enemy jungler is top.
went mid with red level 2? Take his blue and then fight him with your level advantage.
? He's probably farming, hit all 3 lanes before 6 or invade him (depending on your champ).
Jungling is mind-games with the enemy jungler. It's a game of chess. If you know the enemy jungler is top, ping the top laner back and try to take Dragon. If you know he's camping mid for whatever reason, you can stay nearby or put down some wards to help. If you know your bot lane are immobile, you can anticipate heavy ganks and try to counter-gank. If you know he's doing his botside buff because it just came up, you probably shouldn't gank bot.
Importance of ganks:
You should probably focus on ganking 
a lot to shut them down, even if it doesn't result in kills. You should also focus on ganking bot if you can, since a fed ADC is the best thing you can give your team as a jungler. Plus, you can have Dragon pressure by ganking bot. Meanwhile, if you have a tank top, there probably isn't much point in heading there often.
Ally/enemy IQ:
This kinda sounds mean but, if you have an idiot mid who doesn't follow up and can't play from behind, they're probably a lost cause. Mute and leave them. On the other hand, if the enemy top is the special kind of moron that dies to a gank and immediately
back to the wave before you can even leave lane, it's probably a good idea to camp them. You can get very fed very easily. Tilted enemies are also very easy targets to camp.
Ally is flaming:
This is NOT a good reason to gank by itself. Tell him to hug tower and wait. Or mute him. Whatever it is, don't gank JUST because they're crying too loudly. However, sometimes ganks can boost morale.
Let's say Dragon is down, bot is frozen, and top is crying for a gank. You are a free man. You can make a play bot, which is always nice, you can farm your jungle, since the whole thing is up, or you can gank top, which isn't too appealing. But top seems very tilted. It wouldn't be hard to gank and get a kill. Maybe it'll untilt him, especially if you can leave him the kill. It's probably honestly the best move to go for a gank top. A happy teammate is a useful teammate.
Just farm, man.
If you're behind, farm. If it's not important to gank, and you can't foresee a dangerous enemy gank, farm. If no enemies are pushing, Dragon is down, and it would be difficult to dive and get kills, just farm. Do full jungle clears often. You should always be farming. Farm camps between ganks. Don't fall behind because you're trying too hard to make plays to the point where you're 30 cs behind. You'll just lose the game.
It also depends on your champion.
needs a lot of farm.
doesn't, at least until later.
That's a short rundown of the factors you should consider while ganking, but again, the importance of these factors and how to weigh them is just experience. Keep playing, you'll get good and carry those scrub laners.
Source: Jungle main since season 3, highest rank was Gold 1.