You don't need a lawyer to write a valid will. As long as the will meets your state's requirements, it doesn't matter whether you or a lawyer draws it up. Even if you have no legal expertise, you can create a will on your Mac that will hold up in court. The more complicated your wishes for your property, the more work it will take.
Multiple websites offer templates for wills that meet states' laws. Download the template for your state to your computer or cut and paste it into a blank Pages or Word document. Some sites charge for the will template, but some states, such as California, have free templates written into state law. However, customizing or changing a state's template to add your own language may invalidate it. You may be better off with a template that is more flexible.
Relying on Software
Rather than using a template, you can use a software package on your Mac designed to help you prepare a will. Mac-compatible options include Suze Orman's Will & Trust Kit, Complete Family Wills and Standard Legal's will-making software. If you're not a legal expert, research the software before you buy: you may find it is worth paying extra for software with a legal glossary and clear instructions. After you install the software and use it to write your will, the software stays on the Mac for future revisions necessitated by a divorce or the birth of children.
ITunes offers alternatives to buying will-making software. Type "estate planning" or "last will" into iTunes' search engine. You'll find how-to books, audiobooks and podcasts on the subject. There are even write-a-will apps such as Last Will, AutoWill and Write a Will for your iPhone or iPad. Apply the same caution when shopping on iTunes as when buying software. A guide or app that doesn't offer the help you need or doesn't meet state law won't be useful.
Making It Legal
If you leave your will sitting on your laptop, it's useless. Print out the will so it can't be altered and then sign it in the presence of witnesses, who sign too. If your state has any other legal requirements for executing a will, follow them as well. Place the will somewhere secure -- your lawyer's office or a safety-deposit box -- and let your family or your executor know where to find it.