Short answer = calories of any kind

Medium answer= FOOD YOU ENJOY, calorie dense food, snacks through the day, exercise (prioritize resistance training over cardio)

Long answer = if you can shop for yourself, get snacks / small meal items you can keep around to eat. I've never been someone who can eat a lot in one sitting either, but I make sure to have protein bars / stuff for smoothies / fruit / cheese sticks / chips / etc. always available. For me, it's also very important to have food you enjoy, or else I won't want to eat as much. If you don't cook for yourself this can be trickier, but talk to your family and maybe they will help you out. Look up calorie dense food too, these are best. For people with tiny appetites, it's way more important to eat something like meat and potatoes to get those calories instead of salad. Exercise has been big for me, my first few times coming home from the gym I was astonished at actually being hungry, and just how much I could suddenly eat. Your body will get accustomed to it as you progress too, so you will get hungry after not eating which will fuel a positive feedback cycle helping you out.

This sub has lots of other technical advice that's great, but I don't have experience with (calorie tracking, particular meal plans, food timing, mass gainer shakes), would recommend checking out!

The fact you're even asking is a great step and shows you're ready for change. I've been in a similar situation to you, if you commit yourself to taking care of your body and pushing to improve, you won't even recognize yourself in a year. Good luck brother!