I'd probably want to run it through a planer to bring the surfaces down to uniform height. Do you have access to one?

Check with local woodworking businesses if not, or perhaps the wood shop at a local school. Sometimes they'll run it through as a favor.

If not, I'd say the judicious use of a belt sander is your next best option. If you're making fine furniture, stick with a planer, if it's a bread cutting board, you could get away with the belt sander.

Good luck!

Doug's advise is good, and I'd only offer one other suggestion. For $20+ you can buy a good "hand" planer at the local hardware store, while it will take longer than a power table planer it will provide good results. Good luck!