Using the iPhone keyboard is a beautiful thing and it tries to help you out as much as possible. When typing you should just carry on typing even though you know that you have a made a mistake. The iPhone will usually figure it out and by the end of the word is may have even corrected it for you!
At the end of a sentence, instead of flicking over to the .?123 key and finding the fullstop, just press the space bar twice. It will put the fullstop in there for you and get ready with a capital letter for the next line!