Freewheel is the one that can coast. They will thread on either side, if you decide fixed is not for you. But since youre going for both, it threads onto the side that has the same threads the entire way across.
The cog is the fixed gear. It threads onto the side that has the lockring on it. Once the lockring comes off you will see there are two seperate threadings. The larger one goes in the normal direction, that is where you thread on the cog. The smaller ones are reverse threaded(righty loosey, lefty tightey) and that is where the lockring goes.
In order to properly install the freewheel all you need is the right sized chainwhip. For the cog and lock ring you will need the right sized chaimwhip and a lock ring spanner.
Fixed cogs are usually 1/8" wide, so they will need a 1/8" chain, and chainwhip. Freewheels are usually 3/32" wide, but you can get 1/8" wide freewheels. That is probably your best idea.