OpenOffice is a great office software package, including a software for word processing, spreadsheets and presentations - among others. It is mostly compatible with Microsoft Office. Below are instructions to install it.
OpenOffice is so very common on Linux distributions that you might have it preinstalled already. If not, search in your package management for it or use the official installer from the homepage. I don't think there is anything else to explain.
Unfortunately there is no official public pre-release version at OpenOffice.org for Aqua (i.e the graphical user interface and primary visual theme).
One way is to use NeoOffice. This port is based on OpenOffice.org 2.0.3 Code and implements nearly all features of Open Office, including word processor, spreadsheet and presentation.
The installation is quite easy. Like any other dmg-based application, mount it and simply "drag n drop" the application to your Applications folder. That's it.
For that folks using X11 already on MacOS X, there is a Open Office version developed and distributed by the OpenOffice.org team. Here are the steps to install OpenOffice.org using X11 windowing environment.
But beware: This installation is a bit time-consuming and only recommendable if you know what you are doing. There are different flavours of users and opinions why one would use X11 instead of Aqua. I use both ;-)
Before you install, read the instructions carefully and check the requirements. One requirement is X11.app from the XCode Disc (which depends on the MacOS X release) or XDarwin.