In Lenny, WordPress is already part of the apt repository. The downsite is that its a quite old version. So this little howto describes how to install the latest WordPress version (currently its 2.9.1) from its tarball by still retaining Debian’s file locations. Read more here
You need a running LAMP configuration and root permissions to perform the next steps. I’ll use “YOUR.DOMAIN” as a placeholder for your blog domain. Please replace it with your real domain, which is something like “www.kaisblog.de”.