Grats on the new DSLR Neal, they are a fun thing to play with. Get comfortable with the setup you have and make your first add on the flash. You dont need anything too fancy in this department. Check ebay and bhphoto used department and see about picking up a second hand Nikon SB-600 flash, or spring for the newer Sb-700 flash. Add to that a Vello or Stoffen Soft Bounce for another 10 USD, and then a flash cable (to bring the flash off the camera, giving you more creative ability).

The Tamron 90mm is fine. Play with it a long while before upgrading your glass. Your glass will be something that when you do upgrade you want to do it right. Mainly because your glass (lenses) is what holds value. the D90 itself will break over time (shutter wear and tear, accidents, upgrade fever), and its value drops monthly to be honest (all bodies, even pro bodies do!). So get use to the DSLR world, then make a sound judgment about lens upgrades. Don't just buy the 200 dollar 300mm lens because it sounds awesome!!! Wait it out. Learn your subjects you want to photograph, if it's just reptiles, your 90mm will do you for a long while. If you want to move onto birds, you need to consider something more along the lengths of 400mm which cost a bit more and just go WAY up for higher end stuff.

Again, grats, and feel free to shoot me a message if you run into a question.

Cheers