my knees are becoming really frail. where to start?

i am 18 and an antisocial nerd, so this year i have been wasting away and ive been on a downward spiral of depression and thoughts of suicide. falling but not being able to grab anything.so my body has succomb to atrophy due to serious lack of excersize because i sit and use my laptop all day long and fyi its not because of the laptop usage its affected by other variables. so i have noticed that whenever i bend down my knees crack basically everytime i wake up but later in the day i think they stop, it bothers me now becaus i think i have bad knees and what if i cant recover from atrophy just going to make me more depressed.