You will need to wait until the official launch date from Transport for London (TfL), or any other transport operators before you can use your card, which will be announced in the media. If you are keen to try this out sooner, you may be able to register for the pilot on the TfL website.
Once TfL or another transport operator announce its launch, you can use your Contactless card as a ticket if you have a Contactless debit card displaying the Contactless symbol, ensuring you have completed a Chip and PIN transaction to activate when card first received. If you haven't got a Contactless debit card, ask in branch to see if you are eligible.