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If I were you, I'd work on perfecting my technique with the one speed light you have. Don't know what your camera is, but I presume you can fire the SB900 with your on-board pop-up flash. You also need to determine if you want to start with a TTL philosophy, or manual. Mark L above recommends Strobist; David Hobby is a manual guy, and you can't go wrong with that, particularly as a foundation for learning. If you go manual, you can't go wrong with Pocket Wizard Plus X triggers, inexpensive and straightforward, but tough as nails, too. As for modifiers, I'd get a couple of umbrellas, including a small, convertible folding umbrella (e.g. Westcott), a larger 60" umbrella for groups, and one soft box to start, maybe a Westcott 28" or 50" Apollo. You might also check out the Lastolite 8-in-1 umbrella kit, too. Get a couple of good stands, including a C stand "complete" (with a Hollywood arm) for the larger umbrella or soft box. Master these, and you're good to go.