Honestly, coming from a D90, if you handled that camera well, this will not be THAT tough of a transition. There are a lot of things on the camera that you CAN set, but many of them you'll rarely if ever touch.
The whole PSAM thing and adjusting exposure won't be much different than the D90. Many adjustments will in some ways be easier because of the additional external controls on the D300s. Work on understanding the metering and auto-focus systems, and get the ISO, white balance, and picture controls settings down. The rest is details, and you'll pick it up over time.
Grab a book - there are many good ones. Go out and shoot a LOT of pictures. Float lots of questions here if you get stuck.