1) Go here and download Handbrake for the Mac:
http://handbrake.fr/downloads.php
2) Put a DVD into your Mac, launch Handbrake, choose "iPad" from Handbrake's presets. Rip the DVD.
3) Drag the resulting file to iTunes
4) Sync the movie to your iPad.
5) Repeat for each DVD you want to import. Expect to spend and hour or more per DVD.
To buy new movies, use iTunes to connect to the iTunes Store, find the movie you want and buy it. Download it to iTunes and sync with your iPad. Or, you use iTunes on the iPad to buy and download directly to the iPad. You can also download rentals, but rentals are deleted 24 hours after you start watching them.