what do i need to change a one piece to a three piece with a 25 tooth sproket?

Answer 1

You will need all the parts, tools, time and Know how to get the job done.

You could research your specific needs at this link.
http://www.sheldonbrown.com/

An easier alternative is to wander down to your local bike shop and let them take care of it. If you find one of the (mostly friendly) mechanics on a quieter day they will let you watch and spin you a yarn at the same time. For this sort of entertainment people could pay a fortune. But it is usually quiet cheap.

You will pobably pick up a few valuable pointers and be assured that the job will be done right.