Well I'm no expert, but I most certainly follow a very regular fitness regime, and I'll let you know that I used to be about 20Kg overweight, all of which I lost this past year. I also put on a decent amount of muscle. I'm not exactly sure if posting shirtless pics is decent, so I'm not gonna do that, but I have a before / after shot in the old BGSF picture thread.
Anyhow, first thing to remember is to ease into it, and not go all out from the first day, or you'll burn yourself out pretty quick. My primary suggestion would actually be to swim rather than anything else. Not only will you lose fat, but it'll tone your body considerably. An hour a day is what I used to do, but you can start off with an hour 3 days a week.
If not swimming, then there's running on a treadmill, biking, elliptical, or any other cardio machine your gym offers. Remember to go in variation, meaning 2 weeks of each type of cardio exercise, the shock causes your fat to burn off more efficiently.
However this is all just weight loss, for muscle building and / or toning, things are different. But I'd recommend you get rid of excess fat first anyway, so following what I said above would suffice. Also, cut down on empty calories. Meaning no eating cake, or donuts, or drinking Pepsi, or Coke. Stick to natural stuff, no processed meat either. Also make sure that you don't stay hungry, or else it will have the opposite effect. Your body will think it's in a state of famine and retains any and all food given to it if it stays hungry for long periods of time on a regular basis.
I know this is a long-ass post, but it barely touches the surface. If you need additional advice on ANY aspect of working out, from what weights to use, to what your heart-rate should be, down to simply what music you should listen to while working out to get you pumped, you can ask me here, or even PM me, I'd love to help.

Edit: @ cpt krunch13: It's actually inefficient to do muscle building exercises after the cardio. Cardio should be saved for last. Once again, I'm not a professional, but I have almost 3 years of experience, and read a
lot of bodybuilding magazines.