Pretty simple...first, go to
www.straighttalksim.com and order the AT&T-compatible MicroSIM card that specifically-fits the iPhone 4.
Then, find a good, used (or new, if you prefer) AT&T iPhone from the secondary market (eBay), if you don't already have one. Verizon and Sprint iPhones will not work with StraightTalk as they use a completely different technology standard.
Once you have the iPhone and MicroSIM card, do not follow ST's setup instructions, as they tend to be rather "muddy", at best concerning this subject -- instead, there are dozens of
YouTube videos on this very subject that will have your $45 a month iPhone up and running, in no time...