Getting back to my old self

Bella • 21 yr old shameless feminist & lover of the earth ✌🏼️
Alright so I'm going into my senior year of college and I'm starting to notice that the freshman / sophomore 15 is REAL. I want to get back to a healthy weight (for me this is around 145/150 lbs (Personal goal) I'm currently just about 170 lbs , a little less. 
So I need some health advice! I have REALLY bad knees and I am not athletic so workouts would need to be simple or swimming (my ONLY good sport). I can also cook for myself so any nutrition advice is helpful and I would love to get creative with it! What should I do... How should I start? 
Post pics of your progress too if you're on a healthy journey too!