Grant permission to a new user in SQL Server 2014

I'm trying to assign permissions of db_datawriter and db_datareader to a new user in SQL Manager

The following steps were performed:

-- criação do usuário
CREATE LOGIN apostila   WITH PASSWORD = 'qweQWE123!@#';

-- usuário como dbcreator
ALTER SERVER ROLE  dbcreator  ADD MEMBER apostila;

I am now trying to assign db_datareader and db_datawriter to user apostila. I tried the following ways and none worked:

exec sp_addrolemember 'db_datareader', 'apostila'

ALTER ROLE [db_datareader] ADD MEMBER [apostila]

When doing this I get the following error:

Cannot add the principal 'Apostille', because it does not exist or you do not have permission.

That why somehow, the DB is not understanding that the user apostila exists.

If I log in as apostila and fetch CURRENT_USER it will return guest. This implies that if I put guest in sp_addrolemember, ai it accepts and assigned db_datareader to all the guests of the application, and this is the point.

How to assign db_datareader only to one user and not all guests?