I agree a draw knife is a good place to start for most people, but it wasn't for me.
The first items I bought were a rasp and a drawknife, and I've never used my drawknife since then (besides my yew bow).
You have to ask yourself the question what type of bow you want to build.
If you will build a laminated bow (a good place to start for a beginne, like hickory and ash or something like that) you don't need one.
If you will start with building a bow from a stave, than I do recommend you get a drawknife. Be awre though, if you will be using ash a drawknife won't do the job!
If your first bow will be osage or yew or any wood without interlocking grain, then use a drawknife. I used mine on an ash bow and after the first time I switched to my rasp.
if you buy a drawknife, buy a good one from the first time.