I use "Acid" from SonicFoundry
The "Acid" line of products was bought by Sony a while ago... but it's the same product, AFAIK.
The program let's you combine music segments (loops) into a song.
Most/all versions of the product come with a loop sampler, and there might be one with a "rap" specific sampler CD.
Then, after you get hooked on it, you'll be tempted to buy more and more Loop collections -- or maybe find some on the internet for free that some people have made.
you can check out the free version
HERE -- down at the lower-left of the page
google "sonic foundry acid" for links regarding their older products
www.acidplanet.com is the "online community" for acid. Once you have the free version, go here an check out the "8pack" -- which shows what's possible with just the free-version. They are sometimes made by popular/professional musicians and can be quite impressive.