In reply to Joel Perkin:
Suggest you do a search on this forum; it's been debated many times and you'll get lots of diff't views.
Start with a few; most makes are fine. As said above, a fairly small (but not micro) cam - say about 0.5 Friend, is pretty good for finger cracks and I always use that size a lot. Other than that, if you are getting a restricted set for now, people do say that certain sizes are better if you are climbing grit rather than volcanic rock but I'm not sure of that really.
If you are just getting a few, maybe go for those with the widest expansion range. Tend to be a little more expensive (as double axle is presumably more complex to make) but v handy and perhaps easier to get used to when learning? I have BD Camalots but the DMM equivalent is the Dragon.
Wouldn't get anything too large to start with, small (not micro) to medium is probably best to start.