A “carry” is a character who is very weak early game but whose power level exceeds that of other characters around the latter portion of the mid game. They “carry” their team to victory. They typically need a level advantage alongside a gold/item advantage. Gold and (most) items cannot be transferred between players, so it’s imperative that carries get lots of gold and experience early on. Carries can be “hard” or “soft.” Soft carries have a certain amount of utility beyond getting kills. Hard carries are completely useless if they aren’t winning the game for their team.
This is typically accomplished by having the carry do the mid lane, which means faster experience points since it’s a solo lane.
“Ganking” is simply a word for getting a kill outside of a teamfight. You’re off in the jungle killing some creeps and Drow Ranger pops up out of nowhere and kills you? You just got ganked.
Ganking usually happens right around the time the guy in mid lane hits level 6-8, depending on the character. At that time, a good mid will begin ganking, or ask if one of the other lanes needs a gank.
A support is a character who is less dependent on levels and items than carries. Their job is to buy the courier at the beginning of the game, to purchase and place wards, and to purchase items that help everyone on the team (Mekenesm, Pipe of Insight, mana boots, etc). Good support players are skilled at laning with a carry and focusing on denying, pulling creeps (advanced tactic) and letting the carry gather gold. Some supports have ults that are very strong early game, and poor/greedy players will sometimes try to ‘carry’ with them, stealing lots of kills early on rather than trying to let the carry get them. Supports nearly always fall off in power as the game progresses, though, and a real carry is always better than a faux-carry support if the game happens to last a while.
When you’re in the character select screen, you’ll see that all of the characters have symbols on the top right that show what that character is good at. I think they are: carry, support, jungler, durable, nuker, disabler, and escape. I might be forgetting one. If you hover over each of those symbols, it will show between one and three stars. The more stars, the stronger the character is in that category. Three stars in carry means that character is a hard carry. So you don’t really need to ask “what skills make a character a good support?” If the character has a support logo, they’re considered a support.
Creep equilibrium: the primary benefit of fighting near your tower is that it forces the other team to move away from their tower, which pulls them out of position and opens up potential ganks from the mid hero. Early in the game, you can hide under your tower from most heroes because the tower will kill dead anyone who gets too close.