Boxing is a fun sport but it is tough. Some colleges have a boxing club, or you may have to look for a boxing gym around your city. I think the schools clubs are better because they are less intimidating to a first timer. Start slow, learn techniques and combos, then you can progress to harder training and sparring.
After you have learned the basics all you need is a couple friends who also have some training, and then just have boxing matches in your back yard. It's fun, free, and with headgear and mouthpieces usually safe. I stress learning the basics such as head movement, defensive moves, stepping, and alot of time working on form for punches and combos.
I used to do this with my friend who is a 9-3 amateur boxer and has also had a cage match, and it helped me become a much better boxer. When I went back to the boxing club after training in the backyard a few months I parried a 3 punch combo and hit an immediate hard counter left that made everyone ask him "Whoa are you okay?!!"
Its times like those where you make a connection to a technique and your body literally resorts back to your training, and you hit a great technique without really thinking about it, like a reflex.
After you learn the basics and keep practicing them over and over, you will start to make these connections about timing punches, footwork, and feeling "the sweet science" at work. Good luck, be safe let us know how it goes!
Also, like some others have said, being tall and lanky can be an asset as you will have a reach advantage. So you would want to work from the outside whereas shorter guys sparring you will try to get inside and connect with tight hooks. You will want to focus on your jab and your straight(cross) because those will be two of your best assets as a taller fighter.
Be careful hitting a heavy bag without training, you need to have correct form and wrapped hands as a minimum for a serious heavy bag session. Start shadowboxing when you're by yourself. It may seem funny at first but it is a good way to constantly practice punches, slips, and stepping techniques. Start with a jab, jab-cross, and jab-cross-hook combo. Are you right or left handed?