I removed an old furnace from a mobile home and was wondering if it was possible to wire a light fixture hooked up to the wire. The wire connects to a 15 amp circuit breaker. If this is possibe should it use a light switch and fixture or a fixture with a built in switch.

You sure can use the old furnace circuit BUT all the wiring will need to be done in a proper manner just like it would be wired if it were a light circuit when the home was manufactured. You will have to use electrical boxes and grounding that meets code. A light switch will be necessary if you want to be able to turn the circuit on and off OR you can sometimes buy a light fixture with a built in pull chain or other typr of switch. If this space is going to be used as a clothes cupboard or storage area follow the NO BARE BULB rules. Always make sure the glass bulb is covered in case of breakage and a piece of filliment falls on clothing which can cause a fire.