The day my husband and I were to leave the hospital with our son I felt very young and unequipped.  I looked at the nurses who were checking us out and kept thinking, "Your just going to let us walk out of here with this baby!?!?"  Here was this precious little being who was completely dependent and they were going to let us take it home. 
I know, the reality is they didn't really have a choice, but I've since decided that we should have baby vows...you know, like wedding vows.  Then when they place your new baby in your arms you would recite your vows and be given a parents' certificate along with their birth certificate.  And, of course it would be something that you reviewed before the baby-making even commenced.

I, (your name), take you, my Child, to hold you and hug you and kiss you, to love you and to guide you, on good days and on bad days, when you're grumpy and when your happy, when you obey and when you don't, provide for your needs and some of your desires, nurse you when you're sick and watch you run and play, to pick you up when you fall down and praise your best efforts, to foster your imagination and encourage your everchanging dreams, to teach you what I know and learn all I can from you, for as long as i get to be your mommy.

Perhaps it should be revised every 10 years so it could be even more specifc.  That way when they're a baby it would include something about sleep depravation and crying through the night, and stinky diapers and runny noses.  And, when they're toddlers something about potty training.  And, teenagers something about asserting their indenpendence. 
I don't know if there's any way to feel absolutely prepared for when they place that first baby in your arms and you look into their trusting eyes.  There's so much you can be told, but won't understand until the time comes.