Tips on hitting calorie goals??

I'm 19, 5'10 and weigh around 57kg. I've always been skinny and have only now decided to do something about it cause I really am not happy with how I look. I've been going to the gym pretty consistently for the past month or so, however have only really attempted a calorie surplus for the past week. So far im finding it incredibly hard to get down even 2500 calories a day. What I don't understand is my flatmates, who do a similar amount of sport and everything seem to be at a far healthier weight despite either eating less of the same as me on a daily basis.

So far I'm trying to split my day into 4 meals, roughly 600 calories for each with one of those being a mass gainer protein shake (which makes me feel awful) and a few snacks in between. It's been hard even doing this, I can't eat a lot anyway but even finding the right meals to make that are low in volume but high in calories is seemingly impossible. I know all the basics like peanut butter, healthy fats etc etc but when it comes to making 4 easy affordable high calorie meals a day I'm at a bit of a loss, any idea on how I can get in those calories with as little force-feeding as possible??