To remove the freewheel, you'll need the appropriate freewheel remover tool. There are a bunch of them, and the one you'll need depends on your freewheel -- there should be some indentations on the outer face of it where the splines of the tool fit into to wrench it off. You won't need any tools to put the new ones on. Just remember to grease the threads.
Original post sounds like you have a fixed cog on the other side that you want to remove too. For that you'll need a lockring wrench or pliers to get the lockring off (remember that it's reverse threaded, so turn right to loosen). Ideally you'd use a chain whip to get the cog itself off, but you can do it without using the reverse rotafix method -- basically using the frame and chain as your chain whip. Should be easy to Google instructions.