How did you prepare for climbing your first big wall?

I lived under a car before they invented cars and had to eat the cat food cans because the cat food had already been eaten by my Hmong grandmother.  Both of you: get over yourselves. And yeah, living in a car and rock climbing in yosemite is maybe not the end-all of lived suffering.

So to the OP... in addition to the previous good advice re: adequate prep, mental fortitude, etc. I would add: take care of your body up there.  Use a camelback. Eat, even when inconvenient. Baby your hands at night (lotion, triple ointment). Beware the sun.  Maybe find a rest day in there somewhere, if it's a long one.

If you take 5 hours to lead a 5.5 A1 pitch you will most definately fail.

You need some bare minimum level of skill to climb a wall. Taking five hours to lead a 5.5 A1 pitch is indicative of not having reached that level.

This is all good advice. Though some of it is conflicting. You will have to evaluate your skills and proceed from there. 

As a fellow Minnesotan, you may appreciate my first big wall story.

In 1987 my partner and I did Triple Direct on El Cap. It was great time to climb in the Valley. People were starting to go sport climbing, we battled no crowds. Neither of us had climbed anything taller than Devils Tower. We didn't even know anyone who had climbed a wall. Though we were solid 5.11 climbers, we had failed on the Diamond a week before. We both did our first aid climbing, 1/2 a pitch each, the day before we headed up. We both did our first hauls on pitch 1 and 2. We had packed our army duffel bag with not enough food or water but plenty of cigarettes. We completed the route in 5 days, very tired, hungry, and thirsty. We both went on to do much more difficult climbs. Both of us are more proud of this climb than any others we have done. Desire, tenacity, and a high mechanical aptitude is what got us to the top. 

 We are still the best of friends. It was life changing for the both of us. May your experience be as rewarding. Good luck!