this has been convered a million times, use the search function.
briefly though....
mastering isnt some magic bullet for poor mixes, especially if you do the mastering yourself. your mixes should already have depth, space etc in them. if its not there correct it at mix level. you have much more control than you will with a 2buss mixdown (stereo file).
mastering is also concerned with creating a master for the production process. part of this includes correcting any issues within the tracks and ensuring they will sound as best they can regardless of system used however this is only a small portion of the job. you need to try to add cohesiveness to the project, deal with relative volumes, fades and positioning etc, insert codes and create a PQ list and then finally produce a production master and check it for quality and errors.
as such DAWs for mastering in its full sense are limited due to very specific technical requirements. they arent chosen, despite popular misconceptions, because they contain some great mastering bundles or tools in a processing stage.
for the simple processing portion however you can use any DAW. cubase will be fine (nuendo is just the same as cubase but with options for post production and media bundled in primarily pertaining to formats)
in order to deal with issues you will need to assess the audio and decide what it needs then address accordingly. compressors and EQs are generally the most used tools though you are going to come up against all sorts of problems relating to objectivity, monitoring issues being compounded and general abilities to make accurate decisions. mastering is all about why something is being done rather than the what.
one point, a perfect mix will require no processing at mastering, many people forget this. if you mixes arent sounding right then they will be better served having you going over them again to improve. perhaps you may need to address how you record etc as getting the right sounds in the first place goes a long long way towards a good final product