Welcome to the forum!
Well 20" bikes are generally easier to move around and feels really stable on the backwheel, we all call them quite "flicky". Pretty much the same thing for 26" but they're a bit bigger, they both have their pros and cons. Both types of bikes here are more designed for more "jumps only".
Here is a video example:
WHERE you wan't to ride your bike totally depends on you, I only ride my 20" on the street and in cities.
24" bikes are more designed for tricks and trials, like a mix of bmx and trials if that makes sence? You might have seen Danny Macaskill, he's a perfect example of 24" a rider, we call this riding style "street trials".
Video example:
So you need to decide what style you wan't to ride and I'll help you out:)