While weights are a faster way to lose fat (the extra muscle burns calories even when you're resting) there is definitely a place for cardio in your workout routine. No point in being strong if your heart and lungs aren't in tip top shape too. If you're following a weight training program you'll want to limit your cardio workout so that you don't burn away that valuable muscle. When I was in fully fledged weight lifting mode I'd limit my cardio to some stationary cycling before a workout. If your gym has a rowing machine too that's a good one- it hits all the major muscles. Keep the settings light if you're a weight lifter, you'll not want to break down too much muscle. If you're only using weights occasionally (and I have a little workout for that coming soon too John) and doing more cardio, I'd recommend using a variety of equipment (if you're using a gym). A good way to work it is figure out how much time you have in the gym and then split is evenly between exercise machines- step, treadmill, elliptical, row, bike, etc. Leave a little time at the end for some ab work. Sit ups are okay, but crunches are more effective at hitting your abdominals. Three sets of twenty five is good. Another good workout is to get a slant board and do sit ups. Do one normal sit up and then do one where you twist your left elbow to right knee and then another with right elbow to left knee. These three count as one in this workout. A set of twenty five is a good way to strengthen your waist. I'd always say to leave you abs to the end of the workout because they help stabilise your back and if they are fatigued you might be more likely to injure your back. Trust me, you do not want to have a bad back.
Most of your cardio training should be done at a level where you can still speak if needed- you shouldn't be too out of breath. Heart Rate Monitors are a great investment here- they can be programmed to tell you the optimum rate of work to do. The best way to increase your VO2 max (basically the amount of oxygen your body can use) is to do interval training. Warm up on your cardio equipment of choice- I find a bike is good for this but anything will do- then exercise as hard as you can for a set time (I'd do a minute). Go flat out, full effort. Then go real easy for a minute to get your breath back. If you really need more time take it, but try not to do a rest section for more than two minutes. Then repeat. This is hard, hard work and if you're just starting back at training, train sensibly for a month at least before attemtping this. Get a bit of a base behind you (check with a docotor first!) before attempting this. Do as many repetitions as you can manage- if it's only two don't worry. Interval training should only be done once a week. Give yourself time to adapt. You can do your interval training on different gym equipment to work your whole body or you can stick to the same thing each time depending on your personal preferences- afterall, this is to improve your cardiovascular system not your strength.