You cannot do much better than to just get up and read scripture open air. If you are not sure what to say you can just take a book or a chapter of scripture and read it out loud. Scripture is not magic words but you cannot do much better than reading scripture.
You can start off your gospel preaching by reading scripture and then expanding on what that scripture means. Some of my favorite gospel verses to use for this can be found here.
Sometimes when I am open air preaching and I need encouragement I read some of my favorite scriptures. I have them on a sheet of paper that I have tucked into my Bible so that if I am nervous or having a bad day I can remember which ones I want to go to.