I am very lean and how can i become fat in this situation?

Many will be jealous: Generally being lean is a good thing these days--america is the fattest country in the world and this is not something to be proud. If you feel well, eat/sleep well, no aches/pain or significant chronic diseases, then be happy/thankfull that you are lean. As we get older, we will accumulate more fat and reduce muscle mass. So don't become fat; become healthy! consult your doc. Good luck.