Joining a boxing gym is the obvious answer.
If you're looking for alternatives, start working on your cardio. Strength training doesn't help that much in that department. Running 1 hour three times a week will do wonders.
If you have or can get access to a heavy bag, wraps and gloves you can learn much of the basics on your own. You will, without doubt, develop some bad habits, but you can also learn quite a bit.
You can work on conditioning, coordination, speed drills, (functional) strength, etc. on your own. But you'll eventually need a partner to work the punch mitts and to spar. There's no way around this.
Title boxing has a series of instructional videos I torrented long ago that is quite good. I also remember Bas Rutten's Big DVD of Combat fondly, he covers striking in a rather unorthodox fashion. There are many other professionally produced video courses, stay away from youtube crap.