The best thing you can do right now is build up your cardio and conditioning; roadwork, sprints, skipping, etc. It would be best to leave the boxing technique/fundamental development until you are actually in the gym under supervision. That way, you won’t develop any bad habits on your own that you’ll have to unlearn later.
If you are absolutely set on starting before you can get into a gym, there are several online resources available. Precision Striking, Fran Sands, Coach Anthony, and World Class Boxing Channel are all great starting points.