Looking for some advice to improve my health.

xSofiax
I don't really know how to properly explain this, I guess the simple version is I'm looking for a way to gain weight (not quickly or a lot) and a way to improve my fitness/stamina.

I'm really really thin, I think I weight the average of a 12 year old, I checked out some BMI calculator sites (I don't know how accurate they are) but it tells me my BMI is roughly 17.5~ which is apparently underweight (which I kind of already knew).

I've kinda been feeling sick lately and I've read that it could be due to my weight, so I'm trying to improve my health I just don't really know what to do or where to start.

I don't really eat a lot, although I don't starve myself or anything, I never eat anything after around 8pm but before then I'll eat something if I get hungry, I don't purposely not eat or anything but I just do not seem to gain any weight.

I do not really do any exercise either, I think I'm really out of shape since I'm getting tired very easily right now, I'm just looking for a basic way to improve this, something I can do easily maybe with just my friend or sister or something.
Looking for some advice to improve my health.
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