Go to your local bike shop and tell them what you are looking for, and that you are hoping to save money buy buying a used bike from them (people do trade in their older bikes when they upgrade).
Make a relationship with that bike shop so they know you will be their customer, and they will help get you into riding.
I was 406 when I started riding (at 6'3") and was down to 350 (am up to 360 now).
Get a bike, maybe upgrade wheels to accommodate our weight, and ride at least three times a week.
You CAN do this.