If you want to create a valid will, there’s five things you have got to do. You must put it in writing, you must sign it, you must intend that it be your will, you must be at least 18 and of sound mind, and you need two witnesses. Some states differ on the two witnesses part, and might only require one witness or zero witnesses if the will is signed by a notary, but this is the broad-based rule.
This is also one area where the law is pretty strict–a failure to put a will in writing, intend that a document be your will, have the right intent, or sign it–means that in almost all cases your will shall be given no effect. The safety valve is that every state has an intestacy statute that apportions property pretty closely to what people would ordinarily want.