I would start with the Orginal Series, then move on to The Next Generation, and then Deep Space Nine. You might want to watch some of the Voyager episodes before you buy them, because its the weakest of the Star Trek series IMO. Same thing with the first two seaons of Enterprise, but seasons three and four of Enterprise are good (Enterprise wont be out until next year).
Also, all of the even numbered Star Trek films are good, but you should get them all.