I need help with a fitness/workout routine?

I am a male 34 years of age, 5ft. 9in. and my weight is 190. Since becoming an insurance agent last year I don’t get much exercise anymore and it’s really starting to show. I am starting to get that middle age belly and I want to get rid of it before it gets bigger. I was a 34 in the waist but I have had to buy a 36 here recently! What is a good workout for me to do to help slim down the belly? I don’t want six pack abs, just want to trim down my belly. I have some dumbbells, a treadmill, and a workout bench designed for doing leg lifts, sit-ups and related exercises. I have never had any back problems, but I experience some slight pain or “discomfort” in my lower back. Is this normal? Please include as much information as possible because I have NO idea where to even start really. Thanks!