Well theres two parts to your question
Where you should start to actually box.... the best place to start is your local boxing club. Start from the very beginning, learn your punches, technique etc. The coaches there will tell you everything you need to know as a beginner. Theres two halves to boxing - technique and conditioning. You should learn both there.
Where you should start to watch boxing, it's really up to you. Take a look at different weight classes. Some people prefer smaller fighters, some people prefer the bigger ones, some people will watch anything. It doesn't mean them any less of a boxing fan. For Heavyweights, look at the golden era of Ali, Frazier etc. All of which easily found on youtube. If you already know how the bureaucracy works in boxing, you should be able to find the current top boxers pretty easily. Look for the champs at each division, see what they are like, see what styles you like. The sky is the limit as far as material goes. Its really through watching more fights that you find a style of fighter you really are interested in