You can download Nokia Activ Server 2.1 from Forum Nokia website, see URL http://www.forum.nokia.com/main/1,35452,1_1_40,00.html for further instructions about the product and download.
You can evaluate it for 30 days, if you do not have any Network Access Server's available you can always use Nokia Mobile Internet Toolkit (MIT) 3.0 software for testing the WAP GW and real content server connections.
Just set up your WAP GW address into MIT, select Settings -> Toolkit Preferences and create new profile. Select "WAP gateway connection" from Connection Type drop-down menu and enter your WAP GW IP to field "Gateway address". You can choose if you want to use connection-less or either connection-oriented with or without Secure option.
If you're just planning to build a wap site, the only server you need is an http server.
In fact, wap sites are just web sites written in WML instead of HTML.
As soon as you get a place on the web to host files that can be accessibles trough the WEB, You can start create your wap site.
For example, imagine you have a web site (www.yoursite.com). Then your homepage is index.html and can be accessed through: http://www.yoursite.com/index.html. So you can create a wap homepage by creating an index.wml on the same server.
Anybody connected on the wap with a phone will be able to browse your wapsite at the location : http://www.yoursite.com/index.wml.