Take a look at the Oracle site. Sign up for their free magazine, it contains a lot of useful info similar to SQL Server Magazine. Look for Oracle newsgroups, and start looking at the questions and answers. Expect most things in Oracle to be much the same as SQL Server, but also be prepared for different ways of doing familiar tasks, and for a few significant differences. Download a trial copy of Oracle for Windows (or *nix if you run that) and play with it.

Eventually you will know quite a bit about Oracle, but you will have to use a lot of your time to do so. You just need to decide how much time you are prepared to spend, and hence how much you will eventually know. If you want to know Oracle as more than a hobby, then you will need to get a job where you can use it for real.