My immediate advice is, vastly increase your protein intake. Lean protein from any good source is essential for all tissue build-up and repair.
Start a gentle but regular strength-building routine. You don't have to 'go for the burn' to benefit from exercise. Increasing lean muscle tissue will also slightly increase the weight you see reflected on your scales, so don't panic! Muscle weighs more than fat. Happily increasing muscle mass also boosts metabolism.
Weight-bearing exercise is essential for bone health and density, so bear that in mind.
Good luck!