How are tables learned when learning the database?

[Spark]

I am using Windows SQL Server and learning how to create tables. Currently, when creating a table, I set the name of one column as user name and used varchar as the data type. However, when the table is created, I cannot write my own name under the user name column, and it says this data type is not suitable as varchar accepts numbers but not letters. I don't know why this happened, and I got the same result with other data types I tried. What should I do? Which data type should I use? (I am trying to learn everything from YouTube as I do not have anyone to ask for help, so this question might sound a bit absurd or silly, I apologize for that. I would appreciate a serious response. Thank you.)