Weight gain tips?

So I know I could have just google searched this, but I really need tips from people that have actually gone through this.

Info about me:

Age: 18

Height: 1,60m (5’3”)

Weight: 42-43kg (92.5-95lbs) [it varies]

Dimensions: 78-61-89 (31-24-35)

Bra size: 32B

BMI turns out to be 16.8 which is Underweight

How I feel about this:

When I look myself in the mirror, I’m happy with what I see most of the time. The main thing that I’d change is my large thigh gap and small butt. I don’t want my body to turn into a kardashian type of body though. I just want it slightly more healthy looking. I’d also like a fuller bust (something which is hard as it is mostly genetics, but I believe that it is small because of my weight, and it could easily improve in size if I add up some weight).

My goal:

I need to gain up as much as possible until the end of October because I’m planning to start going to the gym from November onwards. The only problem is how?