I really like the calculator here: http://www.health-calc.com/diet/energy-expenditure-advanced because it provides estimates that are not wildly low, and allows you to adjust your caloric estimates pretty specifically based on your activity level.
In terms of activity, I’m a big supporter of folks finding something that’s fun and enjoyable to them, and making that a regular habit. Whether that’s walking or hiking, or swimming, or some sport. Just whatever makes you feel good to have a functioning body, do that.
You didn’t indicate specifically in your post, but it sounds like you’re a woman. Here‘s a guest post from Amber Rogers about the benefits of strength training for women. The benefits apply even if you’re a dude, though. :-D
You’re young and likely still very resilient. I think your long term prognosis is good. Also, this forum topic might be of value to you, started by someone coming from a sort of similar place as you. Good luck.