I made some quick images to show the common ganking paths. Also, since i'm using Colorblind Mode since i prefer those colors, you have to replace Red with Purple.
The Red Jungler can either flash over the wall or walk through the bush behind the Blue Tower and dive a defensive Champion that thinks he's protected by his tower. I haven't seen anyone ward that bush, so if you feel confident in towerdiving, it can make for a quick kill on a very low health champion. You can also gank someone in a more forward position by flashing to a position further north but since the flash nerf, it requires some practice to get flash perfectly right.
This gank is probably the most common of all and the most commonly warded.
If the Blue Player is pushing his lane and advancing into a position past the bush in the water, it's easy to walk up through the bush and attack him from behind.
A very uncommon but still very effective way to gank is through the lane itself. If your ally pushes very hard, you can move through the point between the bushes without getting spotted. If your ally then reduces the pressure and the opponent pushes back, he's in a perfect spot to get ganked from that bush.
You can also only move to the first bush if you have a champion with a strong gap closer.
The reverse is of course also possible.
If you are on the Blue side, the Red Player will usually not push forward very far, which means a gank through the bush at the water won't work, which is why most players gank through the tribush. This of course also means that it's the most common spot to ward for the Red Toplane.
As with the first gank path i mentioned, the reverse is also possible for the blue side. You can move into the small bush near the red golems and make a towerdive from there. However, you still either move through the usually warded tribush or near the tier 2 tower, which means you can be spotted if you don't time it right.
Last but not least, a quick sketch of the paths the junglers can take to reach the lane for a gank, though of course in their own jungle, a lot more movement paths are possible, i.e. the red jungler might move through the tribush into the red towerdive position behind the blue tower.