Starters are not always necessary, depending on the brew, but still recommended. It is much easier to under-pitch than over-pitch, so you generally can't go wrong by making a starter. Plus, unless you are using Wyeast smack packs, a starter will help you determine whether you have viable yeast or not BEFORE you begin your AG brew.
Jamil's pitching rate calculator is a really handy tool for figuring out how big a starter you will need. You can find it at:
Also, you can check the wiki for instructions on making starters.
If you have the time, also make up a DIY stir plate. Makes the job MUCH easier.