Once it's done loading, just type in your nickname and press Enter.
Before entering the chat, you should read this. It may make your stay a bit more comfortable.
You can usually find people here every night (night in the US, anyway), and smaller numbers of people during the day, also. If no one is here, leave the window open for a few minutes before you leave, and maybe you'll get lucky (The room can go from 0 to 10 people rather quickly). Failing that, try coming back on a weekend night and someone will probably be here.
If you have an IRC client like mIRC (Windows users), or smircle (Mac users), you can skip the java and connect to #spite on burghcom.fdfnet.net, or if you can't connect there, try tower.fdfnet.net. Because the java only overlays IRC, this gives you more advanced IRC users extra capabilities. Please note that AOL users cannot use the java chat, and will instead need to download one of the irc clients listed here if they wish to connect. If you want to use mIRC (windows), but have no experience with IRC, I have created a step by step manual on how to install it and connect with it here. Also, broken has created a text on how mac users can setup ircle and connect here.