C172 SOLO STUDENT PLT INADVERTENTLY TOOK OFF ON THE WRONG RWY.

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ON FEB/SAT/02; I WAS ABOUT TO TAKE OFF OUT OF CLASS C ARPT. IT WAS ONLY MY SECOND SOLO OUT OF THIS ARPT AS A STUDENT PLT. I WAS INSTRUCTED TO TAXI TO RWY 8 VIA TXWY F AND TXWY G. I STARTED TAXIING; THEN PERHAPS 20 SECONDS LATER; GND GAVE ME NEW INSTRUCTIONS TO GO TO A DIFFERENT RWY AT A CERTAIN TXWY INTXN. THIS CAUGHT ME OFF GUARD; AND I WASN'T SURE WHERE TO GO; SO I STARTED LOOKING AT MY ARPT DIAGRAM AS I WAS TAXIING. SINCE I WAS TAXIING AND THE NEW INSTRUCTION WAS UNEXPECTED; I WAS UNPREPARED TO WRITE DOWN THE RWY NUMBER AND DID NOT COPY ALL THE INFO DOWN -- ONLY THE INSTRUCTION TO TURN L TO HDG 120 DEGS. I DIDN'T WANT TO HOLD EVERYONE AND EVERYTHING UP; NOW THAT I WAS IN THE SYS AND OUT ON THE TXWY; ON THE MOVE. THE CTLR CAME BACK ON AND GAVE ME TAXI INSTRUCTIONS (PERHAPS HAVING NOTED UNCERTAINTY IN MY VOICE WHEN I RESPONDED TO THE CHANGE). THE CTLR SOUNDED FRIENDLY AND WAS TRYING TO BE HELPFUL. I WAS TOLD TO TAKE TXWY F TO TXWY C; AND THEN I'D BE AT THE PROPER RWY. I FOCUSED ON GETTING TO THE INTXN; AND STOPPED AT THE FIRST INTXN OF A RWY WITH 'C;' THINKING I WAS AT THE CORRECT LOCATION. GND INSTRUCTED ME TO WAIT FOR A B737 WHICH WAS; INDEED; PASSING IN FRONT OF ME. ATC SEEMED TO KNOW WHERE I WAS AND EVERYTHING SEEMED TO ADD UP TO MY BEING IN THE RIGHT PLACE. AFTER I DID MY RUNUP; HOWEVER; I WAS SUDDENLY UNSURE ABOUT WHICH DIRECTION ATC WANTED ME TO TAKE OFF IN; AND I HAD NOT WRITTEN DOWN THE INSTRUCTION. SINCE THE WINDS WERE ROUGHLY NORTHERLY; I THOUGHT THEY MUST HAVE MEANT FOR ME TO TAKE OFF ON RWY 3. I THOUGHT IF I WAS WRONG THEY WOULD TELL ME SO; THE SAME AS IF I HAD ASKED. USING MY CALL SIGN; I TOLD THE TWR I WAS READY FOR TKOF ON RWY 3. THE TWR CALLED BACK THAT I WAS CLRED FOR TKOF ON RWY 3. I ANSWERED 'SKYHAWK X CLRED FOR TKOF' AND PROCEEDED TO TAKE OFF ON RWY 3; TURNING L TOWARD HDG 120 DEGS AS INSTRUCTED. DURING MY TURN; ATC REDIRECTED ME. SHORTLY AFTER THIS; A SECOND CTLR (PROBABLY THE ONE THAT GAVE ME THE ORIGINAL RWY) INFORMED ME IN A SOMEWHAT TENSE VOICE THAT I HAD TAKEN OFF ON THE WRONG RWY. I WAS DEEPLY CONCERNED BY THIS; AND THE IMPLICATIONS. I WAS ALSO MILDLY SURPRISED THAT DESPITE ASKING FOR AND GETTING A CLRNC FOR RWY 3; THAT A CTLR THOUGHT I HAD TAKEN OFF ON THE WRONG RWY. I THEN REALIZED THAT IN MY UNCERTAINTY; I HAD ALSO CONFUSED THE ATC CREW; DESPITE THE FACT THAT THEY CLRED ME FOR TKOF. I FELT VERY BADLY ABOUT THIS; TOO. I DIDN'T KNOW WHAT TO SAY THAT WOULD BE APPROPRIATE AND CONVEY ALL THE THOUGHTS RUNNING THROUGH MY HEAD; SO I JUST RESPONDED GLUMLY WITH 'SKYHAWK X; I SAID RWY 3.' I CAREFULLY AND PRECISELY FOLLOWED TWR AND DEP INSTRUCTIONS OUT OF THE CLASS C AIRSPACE; THEN FINISHED MY FLT; CONCENTRATING ON THE JOB AT HAND. WHEN I RETURNED; I WENT OVER AND OVER THE SIT; I WAS SO WORRIED ABOUT WHAT I HAD DONE. I WAS A LITTLE FRIGHTENED BY THE POTENTIALLY DANGEROUS SIT I HAD PUT MYSELF AND MY FELLOW FLYERS IN. I TOOK THIS VERY; VERY SERIOUSLY -- THIS HAS REALLY LEFT A DEEP IMPRESSION ON ME. I ALSO THOUGHT OF HOW UNHAPPY MY INSTRUCTOR WOULD BE; AND I FELT VERY BADLY ABOUT LEAVING A BAD MARK ON THE PERCEPTION OF GA. I REVIEWED THE FAR'S AND CALLED THE FSS AND A LCL AERO CLUB TO ASK FOR ADVICE. I LOOKED UP WEB SITES FOR INFO AND REVIEWED MY TRAINING MATERIALS. ONE WEB SITE REMINDED ME ABOUT THE ASRP AND ASRS; AND A COUPLE OF PEOPLE SUGGESTED I USE THIS PROGRAM SO OTHERS WOULD LEARN FROM MY EXPERIENCE. SO I'M WRITING; WHICH HAS FURTHER CLARIFIED THE EVENTS AND HOW TO KEEP THIS FROM EVER HAPPENING TO ME AGAIN. FROM MY TRAINING; I LEARNED ABOUT OTHER PLTS GETTING INTO SUCH SITS; BUT I STILL FELL INTO SOME CLASSIC TRAPS. I GUESS I THOUGHT IT WOULDN'T HAPPEN TO ME. NOW THAT I'VE HAD THIS EXPERIENCE; I KNOW FIRST HAND WHAT PERSONAL BEHAVIORS I CAN AVOID IN ORDER TO PREVENT THIS FROM HAPPENING AGAIN; AND TO PREVENT SIMILAR SITS. THERE'S NO SHORTCUT OR WORK-AROUND FOR A 100% CLR UNDERSTANDING OF INSTRUCTIONS; OR FOR CLR COMS. HUMAN PERFORMANCE CONSIDERATIONS: 1) INEXPERIENCE. 2) TIME PRESSURE. 3) NOT WANTING TOSOUND STUPID BY ASKING A THIRD TIME WHAT THE RWY WAS. 4) APPARENTLY BEING AT THE RIGHT LOCATION BASED ON THE DESCRIPTION OF THE INTXN AND THE CORRELATION WITH ATC'S DESCRIPTION OF TFC IN MY AREA. 5) GOT INTO A ROUTINE OF FLYING FROM SAME RWY; TAKING SAME TXWYS; AND FELT FLUSTERED WHEN I HAD TO TAKE ANOTHER RWY. I WAS EMBARRASSED BY HAVING FORGOTTEN WHERE THE RWYS ARE. 6) MY EXPECTATION THAT ATC 'KNOWS ALL' OR AT LEAST MORE THAN I DO; AND IS ALWAYS RIGHT; AND I KNOW RELATIVELY LITTLE. 7) INFREQUENT TRAINING FLTS; SO LEARNING IS NOT REINFORCED. 8) ACCEPTING INDIRECT EVIDENCE OF WHERE I WAS SUPPOSED TO BE; INSTEAD OF KNOWING; DIRECTLY; WHICH RWY I WAS SUPPOSED TO BE ON.