Step 1: Get Xcode
If you want to develop apps for iOS, you need the SDK, which is provided with Xcode. Xcode only runs on Mac OS X (yes, Apple is doing that on purpose), so if you are running a windows (or linux, or pretty much any non-Mac OS X) operating system, you have a couple options:
1. Get a mac, by far the easiest, but it can be rather expensive.
2. Find a friend with a mac, if they are nice, they'll let you use it for programming, you should warn them, however, that programming takes a long time.
3. Give up, Those are your legal options, there are some other options that are either illegal, or are in the gray area (meaning it's debatable whether or not it is illegal), so I am not mentioning them, you want to know them, find them yourself, I won't be responsible.
Now that's settled, onto the IDE. Go to the mac app store (available in mac os x 10.6.8 and later) and search Xcode. It's the first option, click on it. Download it, it's a lengthy download, ~4 Gb, so if you have a slow internet connection you may want to consider doing something else while it is downloading.