A gank is when the jungler comes to one of teammate's lane to help get a kill.
At high levels, if you want ganks you have to set it up well. For example, if you are right in front of the enemy turret, it's not a safe gank. If you're are in the middle of the lane between turrets, then it's all about positioning then. And don't force a gank from your team. It's often a bad decision that can make the game bad in your favor.

Usually you start at the Gromp(A toad) at the bottom left side of the map or the top right side of the map or you start at the golems(Rock like things) at the top left side or bottom right side (Unless you're amumu, then in this case, you go to the blue sentinel, which is a giant golem near the Gromp). Normally, depending on where you go, the bot lane or top lane leashes you. A leash is when the ally laners near the side your jungle helps you do some bit of damage to the jungle camps, but when the monster is at half or low hp, they walk away to give the jungler the full xp, amumu is a different case.

Usually suggested:
1. Fast attacks: Red side
2. Tanks and others: Blue side