Using the markets:
You'll need to get some USD/BTC/Whatever in to Ripple, to do this you need to trust the gateway (say bitstamp) and then withdraw from the gateway to your ripple account.
I looked at Bitstamp but doesn't look like it is a good gateway for me, being in the USA and not wanting to mess with an international wire transfer. Is there another compatible gateway where I can use cash (like Bitinstant) or Paypal or credit card??
Thanks.