I want to lose fats and build muscle at the same time

What's your current height, weight? Do you have prior experience with weight training?
Depending on that you could be looking at a recomp (recomposition) or just a straight weight loss / deficit / cut. Recompositions are slow and involve gaining some muscle while staying near the same weight, so that lose weight is from fat.
If you have a fair amount to lose, you're probably best off doing a cut but still strength training to keep as much muscle as possible - possibly gain some if you're relatively new to lifting.

Either way you'll want to follow a good structured lifting program that incorporates progressive overload. You may need to look into substituting some exercises if your arthritis limits you from doing moves like a squat.

It doesn't seem that people can really lose weight from a specific spot like their belly, the fat tends to come off wherever your genetics and hormones take it from.

Roy1C wrote: »

I've been on the KETO diet for about 7 weeks and have lost 15lbs, mostly from my stomach and love handles. You might consider it. I also strength train and do cardio on an elliptical 3-4 days a week. The elliptical is easier on my knees and legs than running. Best of luck with your plan! - Roy

Keto, even TKD, is less than optimal for gaining muscle. It's ok for the preservation of muscle mass during weight loss, but most studies do not see increases in muscle mass, like shown with higher carb based studies. But if one does want to do keto, it should be a TKD style diet where you consume 30g or more of fast acting carbs prior to your workout and maybe some protein. And then consume protein post workout.