If you just want to sort out volume/dynamics inconsistencies, I agree with the above - get audacity, Reaper or similar DAW, import each song, level match them, apply compression etc.
What is even more important though is having a good monitoring system so that you can critically listen to the songs.
If you have money, Steinberg's Wavelab is a specifically-for-mastering audio editor
There are very few good mastering VST plugins that can be had for free...most people seem to mention 'Ozone' as THE mastering plugin; I've never tried it & it costs a lot.
For mastering, it's not 'mastering' unless you get some one else to do it - thats the whole point of it.
There are members on the forum who can help you out with that (including me)
When I master, I usually go through this signal chain:
Waves Kramer Tape plugin (sometimes), then out to -> He69 EQ (helios-style equaliser) -> LA2A or 1176 style compressor -> SPL GoldMike preamp then back into to the DAW and the PSP Xenon limiter.
I'm building a pultec style EQ at the moment to give me more EQ options and sometimes I will run a mix out to my Revox tape machine if the Waves' plugin isn't doing it for me
Very importantly, I often reference the tracks against commercial tracks of the same genre, to help with tonal and level adjustments.