No idea where to start (gaining weight)

Options
Hi, I am looking to gain weight healthily.

I am 5ft 6 (and a half), 21 years old and weigh 6stone 4pounds.
I was recently ill and lost quite a bit of weight due to not being able to eat. I am usually around 7 stone, have been for around the last 4 years of my life, and only ever really dip a few pounds either side of that. I would like to weigh more than that eventually though.
I have always been naturally quite skinny, but I want to gain some weight.
I have been trying to eat more food, but I really don't know what kinds of food to eat to gain weight healthily. I want to avoid just gaining a belly and that's all.
I've been doing very small amounts of weight exercises per day, such as some squats and sit ups in the hope I will gain some muscle (my stomach is beginning to look a little bit more toned so I'm hoping that's a sign it's working?)
Any help would be appreciated!

Replies