Re: high reps/low weight vs. low reps/heavy weight
1-6 reps per set is generally thought of as pure strength training.
8-15 is hypertrophy (for size)
beyond 15 reps is for generally thought of as veering towards muscle endurance training.
Now, dont think that trying to achieve hypertrophy will make you big like a man, because as a female that's literally impossible (unless youre on testosterone or some form of steroid).
If youre looking to firm up(i.e. build some muscle), i'd say stick to 10-15 reps per set and 3-4 sets per exercise.
Also, keep 80-90% of your exercises free weight. I.e. none of those silly machines. They will do a far better job of building muscle.
As for the weight, that's something you'll have to figure out on your own. Go as heavy as you can push for the number of sets you do. It's not about it being easy. Personally, I love having to push hard through reps.
Also, make sure to incorporate exercises such as Squats and Bench presses into your routine (when you have one of course)
Hope it works out well for you.