All you need to do is buy a Boost Starter Kit (make sure you choose the one that matches the SIM card size you require). Then you can activate your service and initiate the port in of your number from Optus to Boost by calling 1258881, if you can't do that, you can activate online at www.boost.com.au/activate or via Live Chat at http://livechat.boost.com.au
Make sure your handset is unlocked and capable of the 3G frequency of UMTS 850 (like an iPhone). Some phones from Optus are only UMTS 900/2100 compatible. It'll work with Boost but internet will be slow because it wont be able to take advantage of the 850 frequency.
If you let us know what sort of phone you have and if you got it from Optus, we can advise before you go buy a Boost Starter Kit.
Hope that helps
CORY | I do not work for Boost or Telstra.
Boost Care: 125 8881 or 1800 026 678 . Or Live Chat 24x7: http://livechat.boost.com.au