Which pair of dumbbells should I use?

I'm starting an exercise routine with a fitness DVD that combines aerobic and strength training. It says to get dumbbells, but it doesn't specify what weight to start with, 2lb? 5lb? 8lb? etc. I'm a beginner. I just want to tone up and lose some flab. I'm in my 20's, 5'9" tall, 150 lbs and I've lived a sedentary lifestyle up to this point.