Great place to start is with the first ep of series 1, it's called 'Rose'. Introduces you to the 9th Doctor and explains everything pretty well. However the show kind of gets off to a bit of a shaky start (first four/five ish eps), so keep that in mind if you don't immediately take to it. After that it's completely brilliant.
Some people skip the 9th Doctor's reign, I'd advise against that, but if you want to get to the 10th Doctor (Tennant) start with Series 2.
More people skip to the 11th Doctor; starts series 5. Almost a mini-reboot, you won't be confused if you start here. Series 7 is starting soon so if you want to be up to date for that, I'd advise starting with series 5.
If you're looking for a one-off ep to see if you'll enjoy the show, watch 'Blink' which is fantastic episode, with the bonus that you don't need to know much about the show to understand it. It was the first ep I saw and is one of my all time favourites.
Hope this helps and if you have any questions don't hesitate to post them! :)