I suppose you could continue training your upper body, but keep in mind that your primary training goal should be rehabbing that ACL. If you haven’t started yet, let me tell you that REHAB IS HARD WORK, especially as you get more advanced. If you aren’t sweating and tired by the end of your therapy session, your PT doesn’t know what he’s doing and is treating you like a weekend warrior and not an athlete. So keep that in mind and be sure to account for that in your workout schedule; remember that your upper body work is only ancillary.
That said, you can start with the stickies at the top of this thread, and discuss your lifting plans with your therapist.
You must also clearly define your goal. Yes, one goal only, and it must be rather specific. In your case it would probably be to simply slap on as much muscle as you can within a defined time period.
For starters, EAT.
Also, use the search function to find material on adolescent lifting. Be patient, do your homework, and then come back when you have questions.
And don’t forget that your ACL is your primary concern!