Needed:

Soul gem
Summon spell
Soul trap spell

First of all, you need to go get a soul gem(duh!) then get a soul trap spell, amulet, etc. You will also need a sumon spell, scroll, etc.

1st you need to select a soul gem and drop all others.
2nd you need to summon a monster and soul trap it
3rd you need to kill it(there should be a sign that says 'you have trapped a soul')
4th sell it or keep it to enchant your weopons or armor.

Note: you will need a good weapon. Thereare lots of soul gems in caverns and caves where outlaws and bandits and smugglers live and hide thier stash.