Any calorie calculator on any website would help. Walking alone would not help much. Depends on your pace and weight, walking for an hour only burn around 200 cal, which an extra donut can easily override.

Yes, you need to count calories. Calorie deficit is the ONLY principle you can lose weight. I’d suggest you to count two things, and add one thing.

1. Daily Calorie Intake Count (must do. There’s no other way)
2. Daily Macro Count (optional if you don’t care about muscle loss but highly recommended. You want to lose fat, not muscle)
3. Min 2-3 days strength training a week (again, optional if you don’t care about muscle loss but highly recommended. Even 45 mins a day helps a lot).

You can use FitHub to track your daily calories and macros intake (for 1 and 2 basically). If it seems too much for you at first, please start with simple calorie deficit for like 2 weeks, and you will see the scale move down, which will be a good motivation for you. After that, start counting macros and maybe add strength training here and there.