At first:
You can only withdraw from orders that are covered by means of telecommunication or at your home - it's not possible to withdraw from orders made in a shop as they're final.
Second: If -and only if- you have the right to withdraw, you'll be informed on how to withdraw and where to send your withdrawal notice to. As this depends on the type of order, there is no direct answer that can be given here if you don't post at least some information (type of order, eg. mobile contract, DSL contract, cable contract or other kind).
