1-Lift weights. There’s no way around it. In order to stimulate muscle growth, you have to hit the gym. For best results, stick mainly to free weights (instead of machines). They’re far more effective at building muscle.
2-Train for hypertrophy. Hypertrophy is increasing the volume of your muscles, and in order to do this, you should train with heavy weights at a medium/low number of repetitions. I generally go for 8 repetitions, but anywhere in the 8 – 10 range is ideal.
3-Train until muscle failure. On your last rep, your muscles should be totally fatigued; it should be impossible for you to do another rep. If you feel like you could do another rep, you need to increase the weight.
4-Keep pushing. Your muscles will only build when forced to adapt to a heavier weight, so constantly progress to higher levels of resistance. It’s called progressive overload, and it’s exactly what you need to be doing.
5-Keep your body fueled. It takes a lot of calories to maintain muscle. As you increase your muscle mass, you need to increase your caloric intake. If you fail to bump up the number of calories you consume, then your body will be unable to maintain your muscle and your gains will cease. I recommend using the Harris Benedict calculator as a starting point. In addition, muscles require water to grow – make sure you’re staying well hydrated!
6-Eat enough protein. There are a lot of calculations for protein consumption. Since you’re very active and increasing your muscle mass, you’ll want to eat just over a gram of protein per day per pound of lean body mass (lean body mass is your body weight in pounds – pounds of fat on your body). It’s not an exact science, but at 145 lbs and with a lean build, I suspect you’ll need around 175 grams of protein per day – no small feat!
7-Don’t overtrain. You shouldn’t be hitting the weight room for longer than 45 minutes (and in fact, training longer may be counterproductive). Also, ensure that you are resting sufficiently in between workouts, and that you aren’t training muscles that are still sore from a previous workout.
8-Continue with moderate cardio. Almost no one should be doing cardio for durations in excess of 45 minutes (it starts breaking down muscle at around that point – which can actually slow down your metabolism), but especially someone looking to increase muscle size. I’d recommend doing short but powerful 15-minute interval sessions. Don’t overdo it, but don’t skip it either..