change out the hub
it turns the wheel
No, you can just cut your chain with a chain breaker or just take it to a shop and they will do it for you.
Two, but this is not the same as how many speeds the BMX bike in question has. A BMX bike has a sprocket in the front, and a cog in the back and each is a gear, but is considered a single speed bicycle because two gears and a chain are required for transmission of rotational force from the crankset to the rear wheel.
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.
take the cranks off and then take your old sprocket off and then put the new one on and then put the cranks back on.
they can range from 46teeth on a chain wheel all the way down yo a 22tooth chain wheel.
You need the right kind of hub for that. If you haven't, buying a new rear wheel - with the right kind of hub - is probably the sensible option.
Depends on what's keeping it back. If you've got strength left, but can't turn the cranks any faster, get a smaller driver. If you think you could pedal faster but don't have the power, get a bigger driver.
Having one or more spokes loose can cause creaking. If this is the case take you bmx to your local bike shop. Your wheel may not be true (damaged e.g. fractured/not original shape) another reason is that your tire may not be pumped up all the way and the inner tube could be chaffing on the outer casing.
You go fast lean back and pull back... not hard
It's usually not long enough to reach, so no.