When you are at the iPhone desktop, start by selecting Settings. Then go to the third block down in iPhone Settings and select Safari, which is under Phone and above Messages. The very first option in the iPhone Safari Settings is to set which search engine you want in your upper right hand search prompt. You only have the choice of Google or Yahoo, so this is really a matter of preference. The next option down is AutoFill, which is defaulted as off.
When you go into the AutoFill option, you can set Contact Info or Names and Passwords to AutoFill. AutoFill will just help you enter information faster, but this is not always going to be useful. If you want to use the iPhone Safari AutoFill, only use it for Contact Info and not Names & Passwords.