ExcelBanter - Login User Form

Hi
On a userform being used as a Login, I have two textboxes...txt1 & txt2. and
Commandbutton1 = Login Commandbutton2 = Cancel.
My staff member will use txt1 for the UserName & txt2 for the PassWord. The member of staff will use Commandbutton1 to Login once username and
password is correct from a sheet containing list of login details. They will use Commandbutton2 to Quit the Workbook and it should not give the user the option to Save before exiting at this point.

what code do i use to make this work?

When staff type in the correct username and password - this data is
store in sheet (master) - (column A = Username, column B = Password, column C = linked sheet) it will open the worksheet related to that un and pw.

if username or password is incorrect a message will appear saying "Please
check the username or password is correct"

Also the cross ( X ) button on the TOP RIGHT HAND CORNER of the userform
should be disabled - so it stops the user from clicking the X to enter.

Maximum users is 15

I have used previous coding from this forum, but cannot seem to get it to work. If anyone can help me with this, it would be muchly appreciated. Can someone help with code and instruction where i need to input the codes?