Hi. You can cancel any subscription from your account pages. If you also want your account to be deleted, use the online contact form and someone at Spotify can help get this sorted. If you get an automated reply directing you back to the community or to the help pages, make sure to reply to it directly (even though it says not to) to speed things along.
@annabanana6800 - If its just the free 48 hour premium you have signed up for by logging in on a mobile device for the first time then you have no need to worry - the 48 hour trial is non-renewing so your account will simply revert to Spotify free once that time period is over and you can continue to use Spotify on the free plan.
If you would rather delete your account completely, or have set up a recurring subscription (via subscribing or using the 30 day free premium trial) see @Joe 's post above.