Trying to convert a song from one bit-rate to a higher bit-rate will just take up more space and sound virtually the same. You can't really "upconvert" very well.
In OS X, iTunes is super easy. You go to the iTunes tab, select preferences...
Then:
Go to the advanced tab, click Importing... then you can select from the "Import Using" pull down. Then select rate/mono/stereo options if the encoder you are using has any options.
Pop a CD in the drive. A button that says Import CD appears in the lower right corner. Usually it asks if you want to import when you insert a CD, if set in the preferences.
To convert a song you already have, select the song(s), go to the Advanced tab at the top of the screen and simply select "convert selection to (various formats)". It will indicate what format you selected previously in the Importing options.