F / 45KGs, 159CMs / Metabolism unspecified

Hey gang, the metabolism part is unspecified since I am pretty un-sure how it works. Many people tell me different ways on how Metabolism works and it's pretty confusion. I'm pretty underweight. Many people tell me that I naturally look slim, and pretty but this is not the body I want, I wanna gain weight : (
I've been struggling to gain weight, Either it goes lower or somehow stay the same weight, my partner encourages me and helps me out but usually the process drops and the reason is very unspecified, we both don't know. : (
We've been both trying and everything else in the above, thats why I thought It would be a good thing to maybe write here.
If this goes against the rules. I apologise and I'll try my best to rewrite it as best as I can.
What can I do to safely gain weight?
I don't eat much, I'm not really sure how much I should eat (like.. 2-3 meals a day? or, and what exactly?) I am used to eating one big meal a day and that's it, but this whole routine was bothering me