First. Does your mountain bike have horizontal drop outs? If so, then you probably won't need a chain tensioner.

second, are you thinking of replacing your hub? or getting a freewheel

Freewheel goes onto the hub

Here's the cheapest method:
Take chain, brake chain, shorten to fit gearing you wish from the already installed freewheel on you bike. Attach chain. Everything is left alone, you just can't change gears.

Second:
replace freewheel with MBX freewheel. You can use the rear deraillier as a tensioner - if you need it

Third, total fix:
Bmx freewheel, chain tensioner - if needed, 1/8 chain, tappered crank and remove all but one chain ring. Cut or replace tappered bolts to keep chainring on.

It really depends if you need a chain tensioner or not.