Create an Apple ID
The first stage in signing up to Apples developer program is setting up an Apple ID. This is simply a username (in the form of an email address) and a password. It is a single point of access upon which many services can be added, including the Apple Developer account.
To create an Apple ID, click Here.
Become a Developer
The next step is to join the Apple Developer program itself. Go here to sign up. Joining is free and gives you access to documentation, downloads of all current software tools and video guides.
Join the iOS Developer program
The third, and final step is to choose a specific developer program to join. Apple offers three programs: iOS, Mac and Safari. You'll need to select to join the iOS Developer Program to get access to the latest tools and to be able to submit Apps to the App Store for sale. Joining the iOS Developer program will cost $99 (or local equivalent) and you'll need to select the country of residence for you as it requires payment by credit card registered in the same country.
The link to enroll for this is here; Be aware that joining the Mac and iOS programs are seperate, each requiring the $99 charge and getting one does not give you access to the materials of the other.
Information about developing for iOS
Developing Apps for iOS requires you to use Apples developer tools, specifically their IDE, Xcode. Xcode includes a GUI editor, a debugging tool, performance analysis and more. it only runs on a Mac so you'll need one to work on.
In terms of value for money, you will get more computing power buying a desktop rather than a portable Mac. The entry level iMac is a sound choice though do consider upgrading the memory. If you need a portable computer then a MacBook Pro is the next best bet.