I've recently joined a gym, I always thought I didn't need one, but that changed when I moved out and realised that it's pretty difficult to move enough when you don't have many friends or opportunities nearby.
I was happy with my body before joining, which I think is important because you shouldn't attach your self esteem to a number on the scale or your current size.
You see results very quickly as a beginner, and it's tons of fun to build muscle. I've started to mainly hit legs since I'll need them for winter sports, and they've actually decreased in size as I got stronger.
Don't just do cardio, incorporate weight training too, it'll help you see results much faster. Also, it'll help motivate you, because it's difficult to go to the gym just to go run on a treadmill in my honest opinion.
Enjoy getting stronger, improving your endurance and overall fitness, and don't focus on getting smaller or bigger - simply accept any changes that will happen to your body OR do something about those changes if you wish to do so. Eating enough food is very important for your muscle recovery, so make sure you're fine with that and don't restrict yourself too much, thinking it'll make you lose weight, as this won't work in the long term, it's not sustainable.