The screen is small yes but the auto correct on the iPhone is generally quite good so you just have to trust it. Be as precise as you can but try and be reasonably quick too. You will hit the wrong keys a lot and most the time it will pick up what you mean. Just review every text afterwards for mistakes. Click on the mistake and sometimes a list of suggestions will come up, or if it gets highlighted then as soon as you start typing again it will delete the old word and start again.
I'm sure there will be sme YouTube guides on how to use the keyboard.
Granted, I now have an s4 that has a larger screen, but even with my fat fingers the keyboard on the iPhone wasn't too bad..... Still prefer the s4 keyboard etc.
Just give it some time.