Ableton is a very capable DAW not just for live use or DJing.
The simplest advice I can give is to learn the ableton manual first then the accompanying text/video tutorials that come with the software and on youtube.
for mastering, well you could just learn first to build the set of modules involved in a mastering session like your EQ, Compressor Stereo Imager, Limiter. Once you learn how each of them work, line em up and experiment and try to read up a bit online. There are tons of tutorials but I'd suggest learn the modules first.
For synths so you can make your own sounds, google subtractive synthesis which is the most common form of synthesis used in most virtual analogs and which is used the same in Ableton.