I'm assuming you are talking about playing guitar? Learn your bar chords well. Most ska is played on the top 4 strings and the main emphasis is on the upbeat not the down beat. The harmony guide in that link is pretty good, thats not a bad place to start.
Ska bassists are usually pretty creative with their lines. I can't say I'm really proficient at bass myself so I would suggest further digging into the subject. Ska is stereotypically I V IV progressions though so if you can follow that you should be able to pull something together
The best way to learn a style of music, or any music in general, is to listen to it and then figure out whats going on. Figure out the notes of the chords, find the right chord forms for the song, and figure out the rhythm. That goes for any instrument too, if you want to learn how to play ska bass then you should listen to songs and learn the bassline. If you know some music theory you can then disect the music and figure out why its being played too, which will help you write your own stuff.
