I have been shooting full time for 20 years. By far the best advice in this thread is from Frasier. I would add that I don't recommend getting off brand lenses like Tokina and Tamron. They just don't last and the savings is not that much.
Nikon VS Canon is simply a matter of choice. They are essentially the same. Once you choose though you will want to stick with the same brand as the glass is the main cost. Start with fast lenses if you can afford it. 28-80mm or 80-200 Zoom depending if you are more shooting people or landscapes.
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