This one may seem obvious, but it needs to be said: read the documentation. Even experienced DBAs will benefit from a quick read of the pertinent documentation before beginning an Oracle installation. We recommend that you look at the following (at least):
Hardware-specific Installation and User’s Guide (IUG)
Server Administrator’s Guide
Release Notes (usually packed with the media)
The README file, which is usually found on the installation media and contains last-minute information that may not be in the printed documentation.