Help me build my bike?

My search for 'townsend mountain bike' did not reveal a particular brand or model. It sounds like you indeed are missing parts. Contact the place you bought it from and they might send you the parts free.
If you want to rig it in the mean time, get a hose clamp and put it around the seat stem to keep it from falling through.

Slide the seat in to the height you require, keep it at that height by holding it with
your left hand (if right handed) and tightening the bolt with your right.

Also, you could try tightening the bolt a little before inserting the seat pole, this
will hold the seat firmer while you finish tightening.

The bolt that tightens the seat-stem in place is situated at the top of the stem-slot
on your bike.