Quello runs on the Hive blockchain, which means that you need a Hive blockchain account, that you can use on Quello.
There are multiple ways to sign up for a Hive account, a list of them can be found here: https://signup.hive.io/
Personally I can recommend https://esteem.app/signup. It's a free and very fast service. Make sure to save your password, since it can't be restored if you lose it.