For an obese beginner, I would recommend one of Leslie Sansone's DVD's. She has several. On Leslie's DVD's you "walk" (march) in place ... doing some side steps, knee lifts, and kicks. In some cases, you move your arms up and down or work with a stretchy band while you "walk." Nothing is too stressful on the body and she encourages you to ease up if you need to and just walk in place until you are ready to do some of the "extra" moves again.
Another thing I would recommend you do is borrow a few DVD's from the local public library to take a look at them before spending much money. Another option is to view clips of DVD's at a site such as www.collagevideo.com That site lets you view a minute or two for free to help you decide whether or not you want to buy a DVD.
Finally, if you really like a certain program, don't stop doing it just because you can't do it all. Just modify the moves a bit or simply march in place and skip any part you can't do. As time goes by and your fitness level improves, you can gradually do a little more of those tough sections. Remember, if it were easy in the beginning, you would get bored with it very quickly. The best DVD's for you are usually the ones that are a little too difficult for you at first. You "grow into them" as you do them regularly.