Checking if a SQL Server login already exists

I need to check if a specific login already exists on the SQL Server, and if it doesn’t, then I need to add it.

I have found the following code to actually add the login to the database, but I want to wrap this in an IF statement (somehow) to check if the login exists first.

CREATE LOGIN [myUsername] WITH PASSWORD=N'myPassword', 
DEFAULT_LANGUAGE=[us_english], 
CHECK_EXPIRATION=OFF, 
CHECK_POLICY=OFF 
GO

I understand that I need to interrogate a system database, but not sure where to start!