A webserver is were your website will be hosted, you need to buy a webserver from a host, the host that I use is the cheapest and best quality ive ever seen , check out the price plans at www.surpasshosting.com
Start yourself off with a basic package, now the good part about this hosting is that they have pre-installed scripts which means that with ONE CLICK you can install a phpnuke website on your webserver, noramally you have to manually upload it.
So once you have made your phpnuke website now you need to start editing it and uploading graphics to replace the default ones with yours, e.g. a phpnuke theme made by Nukemods normally has the NUKEMODS logo on the website, you need to change this by uploading your own one. To upload/download files to you webserver you need an FTP program , FTP means: file transfer protocal, the best FTP server around has got to be SmartFTP, get it from www.download.com and with that you just connect to your webserver. Back to your phpnuke site..... in your phpnuke site when you login as the administrator you can do lots of things to edit your site, such as news, add forums, make polls and lots of other things.
There are many themes to download from a number of websites on the internet, such as Nukemods for example and you can also download 'modules' which are things like: an MP3 player for your site, a calendar, a shoutbox and many more.
Well thats pretty much about it until you start with all the problems, errors and questions. lol.