gera229, feed/spindle speeds really depend on the material you're using, of course, as well as the properties of the bit. You can actually get bits specifically for different materials (plastic for instance) that can make a big difference in your cut properties. Check out the end mills section of the McMaster-Carr catalog for some good explanations of things like the number of flutes, end geometry, up/down cut, etc.
To start off, I'd recommend getting two general-purpose bits: a square end mill and a ball end mill, which you'd use for roughing (ie removing lots of excess material) and finishing (removing small amounts of material & making finer cuts), respectively. Probably two flutes is fine, and as you experiment with new materials you can expand your bit collection. For feed speeds you want to go as fast as you can, but of course it depends how dense your material is, and whether you're roughing or finishing. For something very soft like prototyping foam you could feed as fast as 100 inches/minute or more, something like Masonite would be more like 30 inches/minute. For spindle speed you want to stay as low as possible while still removing enough material to make a good cut.
It's usually pretty easy to figure out a good range of feed/spindle settings with a bit of experimentation. When you're cutting, pay attention to the size of the chips/dust being created. You usually want little chips/chunks about a 1/16-1/8" in size for a good roughing cut; if you're getting fine sawdust, the feed speed is too low and/or the spindle speed is too high. This builds up heat & dulls your bit.