Preface:
From 2016-2017 I worked on solely lifting weights to try and gain muscle while at the gym. While I was weight lifting I was on a travel soccer team and conditioned for about a week every month, so I guess I was pretty in-shape for my age

At 5’5” and weighing about 115 (17-18 years old), when I first started working out, I gained over 20lbs in about the first 5-6 months of weight lifting and eating as much as could. At this time in my life, I considered myself in the best shape as I possibly could be in.

After leaving high-school and getting into the college lifestyle (@ a stem school) I stopped working out. Fast forward 3-4 years of my life and I lost all the muscle mass I had gained from high-school. In essence, I am skin and bones compared to where I was before, I weigh less than what I weighed in HS (110 lbs).

I am attempting to turn my life around, but I don’t even know where to start. At the moment I am lifting weights blindly (focusing on mostly upper body workouts; chest, shoulder, bis’, tris’, back, etc..) but I feel like my nutrition is **** & my workouts are weak.
I feel as if I have forgotten everything I was taught about a basic healthy lifestyle.

I suppose my question(s) is/are this:
What should my workout progression look like?
What should I be eating to maintain a healthy/gains lifestyle?
What should I focus on first (in terms of muscle groups)?
What should my work our schedule look like? (Assuming I have an empty schedule.)

My goals:
Gain back lost muscle mass.
Try to set a schedule and stick to it
Regain the confidence I once had.
Try to create a consistent diet to follow

Working out from home

If anyone has any input, I would much appreciate your opinions & perspectives.