How to know what database user I'm using in SQL Server?
But I don't know what user I'm currently using


CURRENT_USER will return the current username. See https://docs.microsoft.com/en-us/sql/t-sql/functions/current-user-transact-sql for more detail, and links to other similar functions.

As you can see there are a couple of ways to get the current username:
SELECT CURRENT_USER, USER_NAME(), USER_NAME(USER_ID())
should return your name three times.


  How can I change users depending on the person who's using the database server?


I think you are confusing database users with server logins, which is easy to do. You have explicitly created those users WITHOUT LOGIN. To let someone login as a specific account you need to create a login then create a database user for that login. Something like:

USE [master]
GO
CREATE LOGIN [NewLoginName] WITH PASSWORD=N'PasswordForLogin'
GO
USE [YourDatabaseName]
GO
CREATE USER [NewUserName] FOR LOGIN [NewLoginName] WITH DEFAULT_SCHEMA=[dbo]
GO


in SSMS. There are numerous other options when creating logins (for things like password policy and so forth) and users. See here and here for MS's documentation.