I was charged with DUI on 9/10/2012. I had previous DUI's in 1998 and 2000. Will this count as a first offense?

This will be a first offense for purposes of determining the mandatory minimum (as long as you were convicted in 2000 or at least ten years before the new arrest) but it will be third for purposes of the Judge considering your priors in determining a discretionary sentence and for PennDOT requiring ignition interlock.  Best of luck.  Steve