15 years old- Weight training?

Im an average built guy, not weedy but not exactly Mr Universe. I dont think i can gain muscle that easily but ive never really tried. Im looking for a mix of size/strength, and apparently advised to stick within the 5-8 reps range for this. I have a weight bench and a variety of weights. Im 5,7" and 123 pounds, a little body fat but not noticable, relatively fit. Can anyone reccomend a decent weight (kgs) for me to lift for a good result? Oh and any other tips will earn you love :D Thanks