I've been there, was almost up to 500 lbs at my heaviest. I'm still a work-in-progress, but I'm down 150 lbs and a lot of my joint pain has gone away. Instead of trying to do some big monumental change, start small good habits that are sustainable and work to decrease bad habits. The book Atomic Habits was a great influence for me.

I've gone off wagon the past 2-3 months and have picked up 20 lbs so I'm about to start building my healthy routines again. Right now I'm just logging all of my food in My Fitness Pal so I ca see how many calories/macros I'm taking in each day vs. my Basal Metabolic Rate (what I burn by just existing each). That, drinking enough water, and filling up the three fitness rings of my Watch (standing time, moving time, exercise time) are the foundation of remodeling and from there I can tweak as necessary. Check out the quick start guide in the sidebar at r/loseit.