Write code. Test it. Re-factor it. Test it again.
Look at how other people code, in forums like this, on GitHub repos, open source projects, etc. -- remembering that there is probably more poor code than great code out there. 🙂
Write some more code. Don't forget to test it, re-factor it, etc.
Once you've gotten down the basics of object-oriented PHP, read Matt Zandstra's book "PHP 5 Objects, Patterns, and Practice".
Keep writing code (and testing and re-factoring).