I have a new to me D300s and have a flash question.
When I use the built in flash to take a picture the iso goes up to 3200. I can't figure out why...
I do have the auto iso set but should it bump up that much?
That is normal if you have Auto ISO on. Just turn Auto ISO off with flash indoors.
The D300S was the first of a new Auto ISO plan by Nikon. The D300 and earlier models keep Auto ISO low if you are using flash (because after all, you are using flash instead).
But starting with the D300S, and the models since then, now Auto ISO is set for the ambient, regardless if using flash. And if the ambient is low (where you need flash), then you get a high ISO. The later models do that too (except for a very few of the very latest, D800, D600, and I think the D7100 - these only advance Auto ISO two stops with flash).
But... you can turn Auto ISO off. You may have to be in camera mode P, A, S, M to turn it off however.
If you don't turn Auto ISO off with indoor flash, the incandescent lights are orange, and many fluorescent lights are green. High ISO will see them well. The flash is Daylight (more blue), so mixing light sources will be a white balance headache. Low ISO suppresses the ambient well.