You need 3 things: nutrition, exercise and supplementation. The main (but very simplified) idea is that with nutrition you'll create an energy defecit, with exercise (resistance training in particular) you'll promote fat loss and maintenance of LBM, and with supplementation you'll well... Supplement the other two (by reducing hunger, increasing gym performance, or maybe even promoting fat loss).
For the energy deficit, you're right a desk job and sitting on the couch can take it's toll. You can try to be more active. Running a half marathon every day isn't healthy or necessary, but little things like maybe bicycling to work or the gym can help. That's for the output. Then you have the input: food. For the most part, the answer is simple: eat less calories. Gradually reduce and control your calories until you are losing weight. If you haven't tracked your macros, just doing this is a great starting point. Also food choices are something to look at. I find it helps me to keep it simple: I have pasta, rice, chicken, cereal, coffee, yogurt and maybe some chocolate. That's it. You can get fancy with risotto, different seasonings, vegetable pairings, etc. I just shy away from any of the complex pre-frozen foods and such.
For the gym, simply go hard. Just remember that hard doesn't mean unenjoyable. Focus on total volume and long term development of your weightlifting.
For supplementation, ID recommend you stick with the basics. My general recommendation is creatine, multivitamins, fish oil (take as far away from lifting as you can), ephedrine-caffeine and BAIBA. These should be inexpensive, safe and helpful. If you want you can branch out into preworkouts and such.