How to gain weight? 43kg?

I am 22 yrs old 5ft7 and weigh 43kg - i eat at least 3 meals a day and my blood tests show i have no defidecny in iron etc. I dont exercise for fear of losing any more wight. up untill 17yrs old i was in high competition athletics and ran 5kms a day - i weighed up to 58kg in that time so in the last 5 years since stopping training i have lost over 10kg! any suggestions on foods to eat etc or tips would be appreciated as im very self conscious about my bony and thin frame.