Chimps
Systems stuff - Have a look thru your ATPL Systems such as pressurisation, hydraulics and electrics. These systems tend to be a little more involved than a light twin. Have a look thru turbine engine stuff and in particular research your engine ie Metro / J31 / DASH. I bit of background on its history and design will help you get your head around the array of instruments and indications you will now be seeing
Operational - Know how airspace works above 10000 feet. Look at the routes your company fly to and maybe do a simulated one on paper with regards to clearances / restricted airspace and CORRECT PHASEOLOGY. Be firmiliar with IFR Procedure and CAT Speeds for you type (maybe the same as light twin anyway)
General - Your aircraft may come with Minimum Equipment Lists / TCAS / EGPWS This may or may not be new to you so maybe have a look at this and what CASA say about it all as well.
Have fun