I use reaper and so do a bunch of others, and i see no reason why you couldn't do tracking, mixing, and mastering all in reaper.
Best advice I (and everyone else here) ever got is PRACTICE. Your 2nd mix will be infinitely better than your 1st and so on. My advice (and this is to myself too) is 1) focus on tracking, mic placement and getting the best and clearest sound possible, 2) learn about EQing and Compression (google how to use a compressor and go from there) and keep your mixes simple until you have a good handle on those two things, and 3) only use a few plugins and get to know them (ReaEQ and ReaComp are great and super great for learning). Don't even worry about mastering yet. When you get a good mix save up a few bucks and have a pro do it. I've heard some pretty well mastered albums that were done for $200, well worth it! Oh and experiment with some Reverb and Delay and see what they do to individual tracks and the mix overall.
Keep it simple and fun and let your skills develop, just like your musical skills had to.