This thread is so bait, isn't a simple as 'Eat More'-Obviously that is part of it, but that's like saying to a fat person run a marathon with no training.
People's metabolism are different, I'd suggest making sure you eat something every 3hrs(morn-before bed)Even if just a little snack bar,
Also try stay off fizzydrinks etc that don't help with metabolism, water(while working out) and milk(when not) are great for building up weight.
Try to binge eat abit like pick a certain day say Friday where you don't need to eat healthy binge on big takeouts etc -So you get used to eating more but helps put on bit of weight -Then go gym/workout and build that weight into muscle. Repeat process, stuff like this doesn't happen overnight or in week, takes awhile and to be committed fully to it.