I'm assuming you're using Traktor...
First thing you need to do is within Traktor's preferences, on the Audio Setup page, select the Pioneer DDJ-T1 as the Audio Device.
Next, on the Output Routing page, set your mixing mode as Internal. Set the output monitor to highest two outputs (or if they're labeled Headphone Out, use those). Set the Master Output to the lowest numbered outputs (or Master Out).
On the Input Routing page, you can assign your mic/aux input to any channel (I usually select D), change the L/R inputs to MIC/AUX.
You should be good to go.