you should look on the receipt you got sent to your email address. This should say the username with that you subscribed to Spotify premium! Try to log in with that account. When you see a 10 digit number log in with Facebook!
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LukeRock Star 17
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Subscribing at https://www.spotify.com/premium/ is correct. If you're still not seeing your Premium after this however, I'd recommend having a quick scan of the email accounts you own for emails from ‘email@example.com’. That way you can check the receipt, to determine which one you used to sign up for at www.spotify.com - and log in there.