WESTWIND PLT CONFUSED IAF ON A GPS APCH WITH THE FAF AND BEGAN DSCNT EARLY.

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WHILE EXECUTING A GPS APCH TO RWY 5 AT WAUKEGAN; IL; I INADVERTENTLY STARTED MY DSCNT TOO EARLY AND DSNDED FROM THE REQUIRED 2500 FT MSL TO 1420 FT MSL. AS A RESULT WE WERE TOO LOW; TOO FAR FROM THE ARPT. AS SOON AS THE ERROR WAS RECOGNIZED; I EXECUTED A MISSED APCH AND OPTED FOR THE ILS APCH TO THE OPPOSITE END OF THE RWY WHICH WAS COMPLETED SUCCESSFULLY. THE WIND WAS SLIGHTLY FAVORING THE USE OF RWY 5 AND THE CEILING AND VISIBILITY WERE ADEQUATE TO SUPPORT USING THAT APCH. REQUESTED FROM THE CTLR THAT APCH; AND HE AGREED. I ARMED THE APCH MODE IN THE FMS; ALTHOUGH I AM NOT WELL EXPERIENCED IN ITS USE. MY FO WAS NOT BASICALLY FAMILIAR WITH THE PROCS FOR USING THE FMS TO EXECUTE GPS APCHS. AFTER SETTING UP THE APCH AND RECEIVING VECTORS FOR AN INTERCEPT; THE CTLR GAVE US A HIGHSPD RESTR OF 190 KTS; THEN BECAME UPSET WHEN I LET THE SPD GET TO 160 KTS WHILE LOADING AND CHKING THE APCH IN THE FMS. CTLR THEN ISSUED A TURN AND SAID HE WAS TURNING US TO THE ILS RWY 23 AND MENTIONED 'WAUKE' (ILS RWY 23 LOCATOR). THAT BEFUDDLED BOTH PLTS AND WE SOUGHT CLARIFICATION AND TRIED TO QUICKLY SET UP AGAIN FOR THE ILS. THEN HE CHANGED BACK TO THE GPS RWY 5. IT WAS THEREAFTER ON THE GPS RWY 5 APCH THAT I MISTOOK 'LEESR' FOR THE INITIAL DSCNT POINT. MORAL: WITH AN INEXPERIENCED FO (ON GPS APCHS); DON'T DO GPS APCHS. MORE IMPORTANT: PAY ATTN TO THE APCH MINIMUMS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATES THAT THERE WAS A LACK OF CONFIDENCE IN THE USE OF THE ACFT FMC BY THE FO THAT HE WAS FLYING WITH. THERE WAS ALSO CONFUSION FROM THE APCH CTLR REGARDING THE SPECIFIC APCH THAT WAS REQUESTED. ANOTHER ISSUE THAT ADDED TO THE CONFUSION WAS THE FACT THAT THE DATABASE IN THE ACFT FMC DID NOT INCLUDE THE SAME INFO IN SOME CASES AS THE APCH CHART. THE 2 NM FIX ON THE CHART IS NOT EVEN IN THE DATABASE. RPTR INDICATES THAT HE HAS BEEN TRYING TO GET THE DISCREPANCIES CORRECTED LOCALLY.