All you really need is a text editor :-) but you'll find that using a developer toolkit can save you lots of time until get used to the WAP environment.

But first of all you should visit the authoritative source for all things WAP; WAP FORUM.

Registering yourself with the Nokia WAP Developer Forum and then downloading for instance the Nokia Mobile Interner Toolkit 4.1 should probably be one of your first steps. The toolkit comes with PDF files that will give you references on the WML and WMLScript command syntax and general WAP insight. The Nokia kit requires JRE (Java Runtime Environment) v.1.4.1_02 or higher. A link is available at the same location.

Note that a toolkit might be designed for developing applications for WAP devices in general, not mobile phones in particular. What you make with a WAP developer kit may look nothing like what you get when used from the micro browser in a mobile phone. To be sure things look the way you want, you either need the actual WAP device, such as a mobile phone, or an emulator.

The structure of Nokia resources is somewhat complicated, the browsers, WAP emulators and MMS simulators work as plug-ins to the Nokia Mobile Internet Toolkit. Older emulators like the 7110, 6210, 3330, 3390 or 8310 do not work with NMIT 4.1, start with the ones listed with the toolkit. A must to read is a description of how it works for version 4.0, the Nokia Mobile Internet Toolkit 4.0 FAQ. The toolkit also includes a WAP Gateway Simulator whose logs list the details of the communication with the emulator. Unlike some other emulators, Nokia relies on a WAP gateway to connect to external internet addresses, so either the Nokia (default) or another gateway must be configured in.

Register and check out the Openwave developers' site. Openwave provides the UP.browser which is as far as I know the most widely used browser, especially in the US. Openwave provides a Phone Simulator available for download, after you have registered. Older Simulator versions are also available, for debugging WLM is UP.SDK 4 still the best version. This page helps to understand which Openwave SDK is best for you.

For the Ericsson R320 and the R380 you should register and check out Ericsson's Developer's Zone for developer kits etc. Ericsson does not seem to have a publicly available emulator for the R320.

Motorola has development tools for both WAP and Motorola's VoxML. After you have registered you can download all the software.

WAPmine has a stand-alone application called WAPPage which is a WYSIWYG editor with an XML tree view for editing WML code. Requires only limited knowledge of WML tags.

If you are developing applications for the public, you want to test these applications on as many WAP devices (or their emulators) as possible. Just as you would test your HTML documents on several browsers. Note that the differences between two WML micro browsers will be much more noticeable than the differences between two HTML browsers.