Have you done any strength training before? Cathe has so many wonderful workouts that it's hard to suggest only one or two. The PS series is great. It would be good for beginners and people who are advanced. The only downside is that it takes up a lot of time. Cathe has two total body workouts, PowerHour (more endurance) and Maximum Intensity Strength (strength). If have a DVD player, you can both of those workouts along with BodyMax which is a circuit workout (cardio + strength) on one disc. If you're looking for cardio and strength, there's the CTX series. Each tape has 30 minutes of high impact, high intensity cardio followed by sculpting (working one muscle on each tape). One tape, Leaner Legs, is devoted entirely to lower body.
Cathe has lots of good cardio workouts but most are step tapes. Are you familiar with step aerobics? She's pretty advanced so I wouldn't recommend starting with Cathe for step. The choreography is pretty complicated. IMHO her easiest cardio tape in terms of choreography is Cardio Kicks which you might like because it's like choreographed kickbox/hi-lo. People usually recommend starting with the Wedding video or Step Heat to learn Cathe's style of step aerobics before tackling her more difficult workouts. If you're looking for interval training, Interval Max is great and supposedly (I've never done it) the choreography isn't too complicated.
As a start since you're looking to tone your lower body, I'd recommend either PS Legs or Leaner Legs. Both are killer workouts for the lower body although the latter is a little shorter and more for endurance than strength.