Creating register page using streamwriter

Hi,
I have a project where I am to create a register new user page (with username, password) and write the entries to an array ( in a .txt file). Also once the user has registered, on the login page the streamreader checks the username & password against the .txt file.

I know that a database would be a better option but we have been instructed to use the StreamWriter and StreamReader from a txt file.

I am a bit stuck as to how to get started...any help?

Thanks!