Greetings!
I'm new also... been going to the gyms but I still feel 'new' to the sense that I still don't know what I'm doing.

I remember when I was your age - I tried working out but never kept up with it. It's hard..... but now that I'm 41, I'm just excited that I've been able to continue going to the gym 3X a week for over a year.
Do you drink enough fluids in a day and/or during your workouts? I have this same problem - I just don't feel thirsty. Sometimes I get light headed also.
In most of my cases - it's because I didn't eat or drink enough to get me through the workout.
When I first started out - I worked on the weight lifting equipment and worked myself into a 3 sets of 15 (various weights) and used all the equipment I could get my hands on. Legs, arms, shoulders, abs, back, etc. I only spent an hour 3x a week. Nothing extreme but yet I did get my heart rate up. I've seen the benefits in toned muscles, dropped cholesteroal counts, and I feel better).
Now that I've convinced myself I can do this regularly - I'm changing things up a bit. New gym....new knowledge and trying to find another happy medium to work with - and that's hard to do. I've had some really badly sore muscles after each workout.

Guess I need to back off a bit.
I'm sure everyone is different and you just gotta keep it fresh & new!