There are several factors that contribute to muscle gain. However, in answer to your question regarding reps and weights, here is a general guide.
Reps = 8 reps
Weight = As a general rule of thumb, use as much weight as you can safely lift so that at the end of the 8 reps your target muscles are fatigued, but you are able to maintain your form throughout the set.
Sets = 3 (or at least 2 if you don't have time for 3)
If you are after more of a toned long lean look (like many women are) slightly more reps (say 10-12) reps may be better. The same rule of thumb regarding the amount of weight applies; the most you can safely lift (10-12 times) while keeping good form.
Some bodybuilders will use even less reps (4-6), with very heavy weights to achieve muscle hypertrophy.
The overload principle is the key to gaining muscle. You need to really challenge your muscle's strength (not endurance) to gain muscle. So heavier weights rather than more reps is usually how this is achieved. The idea is that lifting weights (as described above) causes micro-damage to the muscle fibers (this is partly why they are sore after weights). As your body heals this damage, it makes your muscles stronger (slightly bigger) than they were before.
Therefore it is also important to have at least a days rest before you target the same muscle groups with another gym session. Some people will target different body areas on different days (e.g. Monday = arms and core, Tuesday = legs, Wednesday= arms and core Thurs= legs, Friday =rest etc.) However, I prefer to do all body parts on one day and then have a day or two rest before going back to the gym again.
If you want more info feel free to contact me via my profile page or post here.
...as always... this is offered as general information only and is not to be considered as specific advice. :)