MusicMan,
Since I had a lot of the same questions you did when I started, I will try to answer in a way that I hope helps...some here say I am good for giving good answers to newbies because I am so patient!
1st, unless you are only planning on working with the music for your own listening, don't get cheap monitors. If you plan on mixing music to be sent off for mastering or plan on releasing your music to be heard by the public, you will not be able to do good mixes on cheap monitors unless you have a really, really professional ear. I learned this the hard way after trying to use headphones and cheap monitors myself!
2nd, as for the amp, that is a device that basically 'powers' whatever it is you are using it in conjunction with. If you had an amp for a mic, it would power the mic. If you had an amp for a speaker, it would power the speaker. (Example...a regular stereo receiver is basically an amp in that you can plug it in, connect speakers to it and the speakers play the sound. If for whatever reason the receiver didn't have a built-in amp, your would have to have powered speakers that plug into an electrical outlet. (The same goes for monitors...passive monitors do not have any power built into them, so you need an amp to run them. Active monitors plug into an electric outlet and run independently without an amp).
3rd, As for racks, racks are usually effect components that are used to process sounds (reverb, compressor, distortion, etc) and are kept in shelve-like areas like stereo components and hence, the 'rack'. Rack components can also be sampler units and component-only synths (no keyboard).
4th, As someone else wrote, phantom power is for microphones.
Well, that's about all I can offer-hope this helps!
Micro