I joined a gym.. Now what?
See if they offer a free personal trainer session, that would give you the opportunity to talk ti someone about your goals.
I joined 18months ago, had an induction where they show you basics, and settings eg seat adjustments.
To slim down try cardio: treadmill, cross trainer etc. Work through intervals. Try things until you find something you like. Rotate or change things regularly to keep your body guessing.
Don't be afraid of weights, girls don't bulk. Start low until you learn form then increase.
Good luck and have fun