Change Currency

I'm trying to set up a store with the option to view the prices in different currencies and am a bit stuck.

I assume I am to set up a store config for GBP, USD and EUR.

But how do a make it so a user can swap between these prices and how can you set the rate of exchamge between them.

I want to set the base price as GBP and then the other currency prices will be shown based on this multiplies by the conversion rate.

Could anyone provide me with some instructions on how to do this?

Thanks
Ben