Firstly, if you just want a new service with a new number, then follow these instructions, but if you want to keep the number you have, particularly if you're currently with Telstra, then please reply to this post.
Be aware of the two offers that Boost have, the $20 ULTRA offer and the $40 UNLTD (unlimited) offer. You can find the details of these at www.boost.com.au/rates
1. Buy a Boost Pre-Paid Starter Kit, they're available from a number of locations including 7-Eleven, K-Mart, Coles, Big W and Woolworths. Make sure you choose the SIM size that fits the phone you want to put it in.
2. Having ID ready, call Boost on 125 8881 or 1800 026 678. You can also activate by contacting a consultant with Boost Live Chat at http://livechat.boost.com.au or you can activate it for yourself online at http://www.boost.com.au/activate
If you need any further help, just let us know :-)
You have posted on the Boost Mobile Australia Community. I suggest you head over to Boost USA for assistance.