Do a search on stuck stem, its been covered many times before.

You have two completely different problems, a stuck stem and a headset rebuild. For the headset rebuild, I would recommend you visit the park tool site. They have excellent instructions. But you really won't be able to do the headset until you get the stem out.

Also check the seat post. In my experience, if the stem is really stuck (and not an issue of the wedge being stuck or bolt loosening mistake), then the seat post has a good chance of being stuck. A stuck seat post is often harder to get freed up.

I am working on a bike right now that had both issues. Stem freed up with PB Blaster. Post is soaking in ammonia right now. FWIW, its a Trek 500.