jcs or any bike store
The chainring by the pedals and the sprocket(s) by the rear wheel.
25t means that there is 25 of the little spikey teeth on the outside of the sprocket :]
Chain is on the smallest sprocket on the crank and on the largest sprocket on the rear wheel
Not a sprocket only, but you can buy a 23-tooth freewheel that should fit.
Depends. Usually there's some leeway for adaptation, but it's possible that your bike was delivered with the smallest sprocket available for that hub.
There's no set answer to that, it depends on what your preferences are. If you want a bike that's easier to get moving - get a lower gearing(bigger sprocket). If you want a higher top speed - get asmaller sprocket.
It depends on whether you are talking about the front sprocket (the one at the pedals, called a chainring) or the sprocket at the rear derailleur. The lowest gear at the front is the smallest sprocket/chainring. The lowest gear at the rear is the largest sprocket. So if you combine the smallest sprocket at the front with the largest sprocket in the rear you have the lowest gear available on your bike.
The best rear sprocket for your Skyteam ST50 PBR Monkey Bike would be the 32 Tooth Rear Sprocket for PBR 50cc ~ 125cc monkey bikes. If you shop around you can get one for a good price online.
the easiest way is to go to your local bike shop
it depends on what sprocket/gearing setting you have on the bike.
you take the paint and put it on the end of the bike first and work your way up !