OK, I'm biased as I've been a natural bodybuilder for 18 years, but I honestly and truly believe that no personal fitness regimen is complete unless it is composed of both:
(1) Cardiovascular Exercise
(2) Resistance Training
TRAIN YOUR WHOLE BODY:
Without both, your body is not going to be totally fit. There are a lot of misconceptions about weight/resistance training. Firstly, many people think they will bulk up if they lift weights. I have news for you, that takes alot of time and effort unless you are going to get juiced up, which isn't very healthy. Normal people that integrate weights into their fitness routine are not only getting stronger, they are staying off the de-ossification of their bones (VERY important for women after age 30), they are keeping their tendons and ligaments in good order, learning to balance, and believe it or not - BURNING CALORIES!
KEEP IT SIMPLE - BE INSTINCTIVE
I don't like overly anal retentive workouts counting weight used, # of reps, etc. I do at least 30 minutes of cardio every time I go to the gym, sometimes 45, sometimes 60. I either jack the elliptical up to level 16 on a random workout, or it the treadmill on a random setting and interval train (jog/sprint/walk/sprint/jog/etc.) and just rock out on my MP3 player. Then I hit the weights.
I have an ordered system of working my muscles consisting of 4 different workouts: (A) Chest, Triceps, Abdominals (B) Back, Trapeziums, Calves (C) Shoulders, Biceps, Forearms, Abdominals (D) Quadriceps, Hamstrings, Calves
If I simply follow this series I know that I am working my entire body of the course of a week or so, taking the guess work out of training evenly.
NOTE: If I am way busy I do 20 minutes of cardio + a circuit workout on the weights, doing 2-3 sets of one exercise for each bodypart, done in 45 minutes for the whole shebang.
I do not choose exercises to perform, since sometimes the equipment at a gym is taken, its best to be flexible and use what is available, for the sake of giving your body variey, and for the sake of keeping your metabolism up instead of waiting around being lazy.
I do not count reps, I rep to failure every set. Most of the time I will do supersets (2 exercises of the same bodypart done one after the other) or giant sets (3 exercises of the same bodypart done one after the other), 3 sets per series. I warm up, do my heaviest set first while I'm fresh, then break down from there for the next 2 sets.
STRATEGY TO CONSIDER:
When you do cardio, the first 15 minutes basically burns the sugar out of your blood.
At 30 minutes you have burned the fatty acids out of your blood.
Any cardio work longer than 30 minutes is burning STORED FAT. So, if you stop at 30 minutes, you are killing the most productive time of the workout.
That's why I do weights right after cardio, it keeps my metabolism up and allows me to keep burning fat (since I'm training quickly and intensely).
Outside of all this, eat whole foods, cut your carbohydrate intake in half, don't eat sugar or soda or too much bread or pasta. Eat as much fruit and vegetables as you can and make the rest of your calories come from lean meats, organic dairy products, and whole grain breads/brown rice. Drink as much water per day as you can.
If you train and eat like this for a month you will see a mjor difference in you physique and how you feel each day. If you do it for 6 months you'll look great on the beach this summer. If you do it for the rest of your life you will look decades younger than all your peers, and feel good too.