Hi, Megan,
Welcome to the forum, and welcome to soaping!
I can tell how excited you are, and that's great. It is lots of fun.
There was a similar thread a little while ago that might be helpful for you:
viewtopic.php?f=6&t=2171Lots of folks seem to start with sampler packs of bases, micas, fragrances, and a couple of molds and going from there. Hardware-wise, you'll need some glass Pyrex measuring cups for melting your M&P base in the microwave, if that's your method (I like the 4-cup size), mixing spoons, some sort of scale, measuring spoons or droppers for fragrance, a cutting board and cutter for dicing up your M&P base. Stuff like that. There's more on that in the thread referenced above and lots of good advice!
And yes, "four cavities" means that there are four individual molds on a sheet. I am a M&P soaper myself, and use those kind of molds all the time.
I hope you enjoy soaping! It's all about having fun. Don't put too much pressure on yourself to start making amazing soaps right away. It took me about five months before I started making soaps that I was willing to let anyone see!
Don't be nervous or intimidated -- your confidence will go up as you do more and learn more! Everyone here on the forum is very helpful. I'm still learning and constantly asking questions here myself, and everyone really jumps in to help each other out.