How can I gain weight?

I want to not only gain weight, but look noticeably bigger. I'm not going to lie, I am very scrawny and I would like to know how to put some meat on my bones. I need to know a good diet, a good exercise routine, anything to look bigger. I'm not self conscious about myself or anything but I have to admit it is embarring to be 5'8" and only weighing 120lbs. It's not like my ribs are sticking out or anything but I'm just so puny. I need to know the right thing to do because I eat and eat and eat but never seem to grow or even gain weight. Please help.