This is an interesting topic... I'm kind of the opposite in so much as I don't really eat regularly or at all if my wife isn't around (she jokes... and not in a distasteful way due to the mh... that I'd be dead if it wasn't for her)... I'm just very scatty I guess and food doesn't compute unless it's put in front of me. With that said, I'm not underweight (12 and half stone, 6'1).

With relation to going to the gym, do you use protein drinks? Have you researched the type of foods that are compatible with muscle gain/weight loss?

Did a very quick google search and came across this site (no idea if it's fishing for customers or what not, but has a calculator) Bodybuilding.com - Calculate Your Recomended Calorie Intake!

Fitness regimes do require some serious restrictions to diet... fast food is obviously a no go and you've identified that... but I guess it's going to take some research... even if it means looking for other community forums that specialise in fitness.

As for motivation with regards to depression... well yeah, heh... that is the biggie. Something my wife has noticed with me... and so I can only talk from personal experience... is that once she gets me out and to a place, I will get on with if it as best I can... so perhaps look at it in this way: It's not getting motivated to being at the gym... it's motivating yourself to travel to the gym.. motivate yourself to get changed... motivate yourself to exiting into the main area of the gym... break it down into steps, that way it makes it smaller and just a little thing to do at a time.

I hope some of that is helpful