Free Lance Writing

Does anyone know anything about free-lance writing? I looked some things up on the internet and found companies that pay per article. If these things are for real, I think it could be really good extra income for us. Plus it could open the door for a career in writing. Every company I've looked at request a writing sample. Most of what I've written are school papers and I don't know if that would be appropriate.

Anyone know anything about this?