Ok, at this point and time in the process put mastering COMPLETELY out of your mind. That is the last and final step in the process and is usually done by someone who specializes in that area....what you need to concentrate on right now is mixing. Mixing involves setting the level of all the tracks so they compliment each other. During this phase you can also add compression, eq, and other effects to create the overall desired result.
You obviouslly have a computer....google a few mixing tutorials specifically for Reaper. There are Youtube videos and a wealth of knowledge if you just look.
Welcome aboard and good luck!
...we have reached a time where infinite growth is colliding with finite resources. We are in trouble....