Laws regarding creating Wills vary by state. However, in general, the following must be true:
You must be a legal adult of sound mind.
You must sign the Will. This can occur even if you're on your deathbed, so long as you're of sound mind and have witnesses.
You must have two adult witnesses of sound mind sign the Will unless you live in Vermont, which requires three witnesses. These witnesses can't be in the Will to avoid bias or coercion.