I can see and feel my ribs by looking in the mirror. Should I gain more muscle? If yes, then how?

Some would b jealous. During time of famine, having a bit of extra fat was a status thing (wealthy enough to have food), but it is not so these days. In your case, wanting to gain muscle is a good thing, to a point. So, if you are eating a healthy, balanced diet and desires to add some weight-bearing exercises to gain some muscles, go for it. Easy comes/easy go, so take time and do it well--doing it for you. Good luck.
Sure. Gaining muscle is good for everyone, it will give you more spine stability and strength you can start doing weights at the gym at least 3 to 5 times a week. You should work mayor muscle groups and alternate them. For example monday chest and back, wednesday legs and friday arms and shoulders and incorporate some abs exercises on each day. If you can afford going to the gym go to www.Bodyrock.Tv.