First, don't be scared to gain too much muscles - it's not that easy, unfortuneately
Then, i agree with Johnathan - to a certain degree you will gain muscle with the average diet, but beyond beginners level, you won't. But that's a good thing for you
I would either focus on strength, meaning heavy weights and low reps <5. That way, muscle mass increase should be minimal. But don't do so if you've just started working with weights.
Or i'd go for high volume, many exercises and sets, high reps, and often, that way you build a good fitness and toned body, without bulk.
Only doing 15 reps instead of 10 won't do the trick, i think, because some people actually gain better with 15 reps. It depends on the muscle, which fibres are dominant. Lower body generally responds well to high reps, upper body to low reps, but of course that varies with the individual, too.