I've just started; what do I need to do? I want to change my lifestyle.

I'm a 28 y/o male weighing about 90-92 kg. I had severe scoliosis as a child, but managed to 'fix' that mostly when I was younger. I have a large torso with shorter limbs. I have mild lordosis and rounded shoulders (+ a neck hump) from doing anything that I wish I hadn't - slouching, too much screen time, etc. - It's making me insanely insecure.

I looked at myself in the mirror today and decided I couldn't live like this anymore. I need to change my life. I WANT to change my life and lifestyle. I know I can fix most of it by paying attention to what I eat during the day + working out, but as much as I'd love to eat better, I do not have the resources or money to go out and buy whatever.

I also have NO idea what to do in the gym. I started a gym membership last week, and decided to check it out with one of my close friends. I got a bit overwhelmed, but hey... I was there! That's a good start, right? Mind you: I don't want to be the biggest guy on the planet, but I would love to lose my skinny-fat look. I want to feel good and secure in my own body.

I have the opportunity to go to the gym about 3 times a week: Mondays, Thursdays, and Saturdays. What would be a good way for me to start? I'm thinking;
Mon: Arms & Chest + Core & stretch exercises for my lordosis
Thur: Abs & Back + Core & stretch exercises for my lordosis
Sat: Lower body + Core & stretch exercises for my lordosis

What would be ideal for me? Am I thinking in the right direction?