INEXPERIENCED COPLT DISTR AND AUTOPLT OFF WHEN THOUGHT TO BE ON CAUSES DEV BELOW ASSIGNED ALT.

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ON DSCNT; MY COPLT AND I WERE CLRED TO 8000 FT. WE WERE AT 10000 FT ON THE ROCKIT 3 ARR. THE ROCKIT 3 HAS MANY FIXES AND OUR GPS CAN ONLY HOLD TWO AT A TIME AND MUST BE CONTINUALLY PROGRAMMED. WE WERE NAVING DIRECT AS PER A PREVIOUS ATC CLRNC. WHEN WE WERE CLRED TO 8000 FT MY FO ACKNOWLEDGED THE CLRNC AND I INITIATED THE DSCNT ON AUTOPLT. AT SOME POINT IN THE DSCNT MY COPLT; WHO HAS LESS THAN 100 HRS IN THE ACFT AND NO PREVIOUS JET OR TURBINE EXPERIENCE; BECAME CONFUSED ON THE PROGRAMMING AND I THEN ATTEMPTED TO ASSIST HIM. THIS IS WHEN I NOTICED THE AUTOPLT WAS OFF AND I DSNDED 800 FT BELOW MY ASSIGNED ALT OF 8000 FT. I IMMEDIATELY RETURNED TO 8000 FT. I BELIEVE SEVERAL FACTORS CONTRIBUTED TO THIS EVENT: FATIGUE; WE WERE AT THE END OF AN 11 1/2 HR DAY; 2) LACK OF COM ON THE CREW'S PART; AND 3) LACK OF A COMPANY CRM PROGRAM; CAUSING THE CREW TO NOT STICK TO THE PRINCIPLES OF CRM; FINALLY; I; AS THE CAPT; FEEL THAT I TAKE ULTIMATE RESPONSIBILITY OF ENSURING THAT MY COPLT WITH HIS LIMITED JET EXPERIENCE SHOULD FEEL COMFORTABLE WITH THE ACFT'S EQUIP; AND THAT I SHOULD HAVE BEEN MORE VIGILANT IN FLYING THE AIRPLANE FIRST. I HAVE HAD MANY COMS PROBS WITH MY FO'S BECAUSE THE COMPANY I WORK FOR DOES NOT HAVE A CRM PROGRAM FOR ITS 150 PLUS PLTS. EACH CAPT DOES THINGS DIFFERENTLY AND I AM CONSTANTLY ASKED BY MY FO'S AS TO WHAT I EXPECT. THIS SHOULD NOT BE. THE COMPANY SHOULD STANDARDIZE ITS OPERATING PROCS. I WILL ENDEAVOR TO BE MORE VIGILANT OF MY FLYING AND NOT ASSUMING OF MY FO'S ABILITIES. I BELIEVE THE LACK OF CREW COORD TO BE A SERIOUS PROB.