First, way to go on taking control while you're young! It's much easier to make lasting changes at your age. Also, it's great that your GF is wanting to make changes as well- it's always much better when you have a supportive partner.

My #1 piece of advice is, TRACK YOUR FOOD! It sounds like you are on track with your workouts, but food is 90% of weight loss and if you don't track it (at least in the beginning), you are likely to see little progress. It always amazes me how much I have underestimated the calories in what I am eating. Once you start tracking (most of us here use My Fitness Pal), track what you are eating now for a couple of days. You will likely be surprised by how many calories you're taking in. Then you can start making adjustments!

I don't know your stats, but you can figure out how many calories you should be eating in order to maintain your weight here: http://www.freedieting.com/tools/calorie_calculator.htm. Once you have figured that out, you can easily figure out how many calories you need to eat in order to have a calorie deficit. For example, my maintenance number is 2200 calories (BMR x 1.25, for "sedentary"- you can find those numbers in the FAQ in the sidebar on the right), so in order to lose 2 lbs per week, I need to eat 1200 calories per day (1000 calorie deficit per day = 7000 calorie deficit per week. Since a pound is 3500 calories, I will lose 2 lbs per week if I hit my deficit every day). It might sound like a lot of math, but you only have to figure it out once to get started!

Have you read the FAQ? There's a great section on getting started that seems like it would be perfect for you! https://www.reddit.com/r/loseit/wiki/index#wiki_getting_started

Good luck! You are already on the right path, you just have to figure out the details and form an eating plan. :)