Hello guys. Recently I've been inducing a lot of change

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Hello guys.

Recently I've been inducing a lot of change in my life - a lot of improvement - and I've decided to use some of the time previously spent with vices working out, and hopefully becoming a /fit/izen myself. My goal is not to become a body builder, but I want to be lean and muscular enough to look good, and to be the fantasy of some chick out there. I already read some things about both workouts and eating healthy, but I have a few questions.

My baseball stats first:
>23 yo
>53.7 kg
>1.75m (5.7 ft)
>Shitty posture
>The least amount of muscle a "functional" human bean can have
>The least amount of fat a non-anorexic, non-african person can have
>Haven't touched sports for like 3 years, never was a sports guy.
>No gim/cardio/weight lifting experience
>About 5"

My biggest concern right now is in my weight. I'm thin, and my diet, while not complete trash, is not the best either; I know very little about food and where what I need comes from, and I feel it'd take some time to get that down. Should I leave working out aside for now and just focus on eating healthier and gaining some weight, or should I just jump on both things at once? Pic related is me currently, btw; no bully pls.

Second question is: should I work out even if I'm sick (with a flu, or something like that), or should I rest until fully recovered? What about medication and the like? How bad is it to miss a day because of something like that?

Other than that, any advice would be appreciated. I'll also be recording my progress as I go, so I can make a "made it" thread one or two years from now; but one step at a time.