How do I use a freewheel as a single-speed? Can I disassemble a free wheel sprocket and use it as a single speed sprocket or I have to buy a single speed sprocket?
There are multiple options.

Just use one cog and don't change gear.   Cheap and easily reversible.
Remove and store your rear derailleur/shifter/wires, and then trim your chain to length so it only suits one cog.  This may require a chain tensioner and risks dropping the chain off, if the chain line isn't good.
You can get single-speed kits that fit on a freehub, essentially a cassette with one cog and spacers for either side.  Since the horizontal placement is adjustable, you should be able to get a better chainline.  If you only have a freewheel, this is not an option.
The expensive solution is to get a single-speed wheel, or rebuild your existing rim onto a single-speed hub.   This may give you a flip-flop setup where each side of the bike has a different gear, or the choice between a fixed-gear and a single speed.

It really depends if your frame has track ends (probably not if it has a derailleur) or some other way to set the chain tension.

Remember you still absolutely need working brakes, front and rear.  Don't remove them in your quest.

Personally I just ride with my gears - the 1935/6 Tour de France riders petitioned long and hard to be able to use gears!