New woodworkers are often intimidated by the prospect of cutting their own lumber to length. This is made worse by the constant dire safety warnings about how saws will take your fingers off (which they will), and the fact that as a society we condition women to be timid and to think of machinery as "guy stuff." As more women come into wood working, they're having to confront this issue head on.
The easy out is to buy all of your lumber at a home center and ask the staff there to cut your material. Unfortunately, these people don't really know any more what they're doing than you do, and they have a much smaller investment in getting it right, because they aren't building the furniture for their own home. So what is the aspiring new woodworker to do? Cut her own lumber, of course. Fortunately, this is easy, safe and inexpensive.
If you're buying sheet goods, you absolutely need a circular saw. Don't cheap out on blades, either. The quality of your blades determines the quality of the cut. The best way to learn how to use it is from the Pink Toes and Power Tools post that I have linked to in the past on Cutting Sheet Goods with a Circular Saw. Also, be sure to cruise around Kristen's site, she has lots of cool ideas.
If you're buying actual wood, you can do your cuts with a hand saw. In fact, you can bring a hand saw with you to the lumber yard in case you need to cut something down that is too long to fit in your vehicle. More than once I've been seen at the home improvement store with a hand saw shortening lumber to fit in the trunk of my old taurus. There are plenty of good sources for information about using hand saws. My favorite so far is a video by Chris Schwarz on general hand tool use, which I wrote about in Learning to Use Hand Tools. I've purchased the video, watched it, and can confirm that it's a really great introduction. If you prefer books, and don't mind having to do a little mental conversion from British woodworking terms, I also wrote about Rob wearing's excellent book Getting Started in Woodworking. Both of these books cover more than sawing, but it's all useful information, and by learning how to use simple, quiet tools you will be much happier with the quality of your finished work. You'll also be happier with the amount of money you saved.
So remember: don't let the idea of cutting your own lumber intimidate you. Building furniture is no harder than cooking a meal, and no more dangerous. If you can do one, you can do the other.