To develop "native" applications for iOS, you are required to buy two things and download one piece of software.
1) An OSX computer. Costs between $600 and $4000.
2) An iOS developer license. Costs $99 a year.
3) XCode. Free.
Your other option is to learn how to develop HTML 5 and JavaScript web pages that have the look and feel of an iPhone App. This requires certain meta-tags in the HTML and adhering to iOS centric practices. This cost nothing but the ability to host the required HTML and JavaScript. Which can be as low as $30 a month for hosting.