Hey Caledony,
Welcome to Perth! You'll find plenty of windsurfers over here and everyone is pretty friendly. Your best bet is to be prepared to do some big kms in order to get the most out of your stay. If the forecast looks good I'd suggest heading to Coronation Beach (Geraldton) and staying there (you can camp at the beach - there's a campground right there). If the forecast is in it's probably the perfect place for someone with your ability to spend 15 days.
Lancelin is also a great place and there's a great backpackers place to stay:
www.lancelinlodge.com.au/ - I haven't stayed there for a few years, but it's high quality and of course populated by cool people.
In Perth you'll meet loads of local sailors if the seabreeze is in. Pelican Point and Lucky Bay on the river can get pretty busy (in a good way) once it's over 18 knots from the SW.
Enjoy your stay!