Couple of sites I can refer:
- Start with wordpress codex http://codex.wordpress.org/
- http://net.tutsplus.com you may find good starting point there.
- Search Youtube you can find some good ones there too.
- And if you feel like paying for it try some tutorial series from http://lynda.com
- Search google. I think its the best way to go 🙂 there are hundreds of sites out there publishing written and video tutorials for beginners and advanced.