FLC NEGLECTS TO MOVE XPONDER FROM STANDBY.

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AFTER DEPARTING RWY 22R AT EWR WE WERE HANDED OFF TO DEP CTL AND ASSIGNED A HDG OF 220 DEGS AND TOLD TO CLB TO 10000 FT. WE WERE ON THAT HDG FOR ABOUT 20 MI AND NOTICED THAT THE CTLR WAS NOT TALKING TO US. THE PF (CAPT) NOTICED THAT THE TCASII DISPLAY WAS 'BLANK.' HE QUERIED THE CTLR ABOUT OUR HDG ASSIGNED. THE CTLR WAS VERY BUSY WITH AN ACFT THAT WAS 5 MI OFF COURSE. THE CAPT REALIZED THAT THE XPONDER WAS STILL IN THE STANDBY MODE. HE TURNED IT ON AND THE CTLR TOLD US TO TURN R TO 360 DEGS AND CLB TO 11000 FT IMMEDIATELY. WE RECEIVED A TCASII RA AND VISUALLY IDENTED THE TARGET. I; PNF; WAS RESPONSIBLE FOR TURNING ON THE XPONDER DURING THE BEFORE TKOF CHKLIST I READ THE ITEM (COMMAND) AND RESPONDED BUT FAILED TO TURN IT ON. DUE TO MY LACK OF EXPERIENCE AS A FO; I WAS AN FE FOR THE AIRLINE FOR 4 YRS. I HAD NOT DEVELOPED A 'FLOW' YET FOR THE CHKLIST ITEMS. I AM ON RESERVE DUTY AND THE LAST TIME I FLEW WAS 7 DAYS PRIOR. I HAD A TOTAL OF 28:40 HRS IN TYPE AND AS A FO THIS LACK OF FLYING WAS A CONTRIBUTING FACTOR. I WAS STILL 'RUSTY' BECAUSE OF THE INFREQUENCY OF REGULARLY FLYING TRIPS (ALL RESERVE PLTS). I THINK THE COMPANY SHOULD BUY ME A COUPLE OF MONTHS WORTH OF TRIPS TO BRING ME UP TO SPD AND KEEP ME FROM GETTING COMPLACENT. WE TALKED TO A TRACON SUPVR ABOUT THE INCIDENT AND HE SAID HE WAS VAGUELY FAMILIAR WITH SAID INCIDENT. HE SAID THERE WAS NO VIOLATION OF SEPARATION.