This is a fairly complex legal question. Depending on the size of your estate, you probably want to hire a lawyer. I believe you can find a number of legal services that will do flat-rate wills for (I think) a couple hundred bucks. This is probably most important if you have kids or a lot of assets.

If you don't have kids, and you don't have a lot of assets, or you seriously dislike lawyers, you can put together a will from a pre-made form and get witnesses (who are not beneficiaries) to sign the will, ideally in the presence of a notary public.

Legal documents are often highly technical and vary meaningfully from state to state. Some of this is defensible - you'd be surprised how things can get messed up - and some of it exists to keep lawyers employed. If it's really important - if you are not a mostly asset-free student willing various odds and ends to family - it's probably worth having it done professionally. Five or ten minutes on google searching for estate lawyers in your area will probably do the trick; or you can look for the fixed-rate will deals from bigger businesses that I mentioned earlier.