the easiest way is to go to your local bike shop
It will if you have bmx (or bmx style) cranks.
Not a sprocket only, but you can buy a 23-tooth freewheel that should fit.
No. a 9T sprocket will only fit a cassette hub, and not freewheel hubs.
i would assume you have cranks with a built in sprocket, if this is the case, you can't
change out the hub
There are two different hub designs for BMX, freewheels and freehubs. You need to know what you have if you want to replace the sprocket by the rear wheel.
a 25 tooth sprocket will fit on a 9 tooth driver
You might have to either lengthen or shorten the chain, depending on if you're getting a bigger or smaller sprocket.
Of course it will, but in my opinion freewheels arent the best
To determine if a 25-tooth sprocket will fit your cranks, you'll need to consider the crankset's compatibility with the sprocket's mounting standard. Check the bolt pattern and spacing of your crankset against the sprocket specifications. Additionally, ensure that your chain is compatible with the sprocket size. If all these factors align, then the 25-tooth sprocket should fit your cranks.
Yes Pretty much all 3 piece cranks should fit any sprocket you buy for them.
No, there are several different diameters to choose from.