I am trying to create a Notepad application in C# that uses a SQL database. How to cross references users with another table?

I am trying to create a Notepad application in C# that uses a SQL database to allow users to log in and access previously-stored notes. How would I go about this? I am trying to develop an application that will work locally on someone's computer but can log in with different usernames and passwords to store different notes. For example, bobPersonal123 has 20 different notes, where each note has a Title, Message, and Picture. Bob also has a work account, bobWork123, that has a different set of notes, let's say 15, each with a Title, Message, and Picture. I know how to use a separate SQL table for each aspect, logging in with different accounts and storing 20 different Title, Message, and Pictures, but how would I be able to allow a specific user to access a specific table of notes once logged in? Would I essentially need to create a table within a table? Or is there a way to create a column for bobPersonal vs bobWork that contains some sort of reference to another table? If so, how do you create references to other tables?

Sorry if this question has a fairly obvious answer as I've only just begun using SQL. Thanks in advance.