Was mentioned above to warm up, stretch, cool down. Do these things and make sure you're using good form and you just can't go wrong at your youthful starting point. Experiment with your diet. Make sure you are eating a lot of clean food, especially if your gains seem to be coming too slowly. At your age lots of guys hit each muscle twice per week. Use this time to figure out if you personally grow better with higher reps or higher weight low reps (most likely a combo of the 2 will produce the best results). Focus on getting a GREAT pump every time you're in the gym and get in the gym as often as you can. FIND the time. You're in an exciting place for weight lifting. You WILL see the best gains of your life right now if you eat hard and workout even harder. Do whatever you have to do to find motivation to lift most every day. Work out partners with the same goals as you and that NEVER miss a workout. Soon you'll have some much more specific questions and then you find some folks to ask.......like the great group of guys on this site. The guys on here know how to work out and sacrifice for the body they want.....and they will GLADLY pass their passion along to you.
Good luck my friend. Lift , sleep, and eat HARD