VB.Net Global Variable

I have create a module namely globalvariable and want to create a global variable namely SQL type string when i am doing this it is showing errro on Global:

global SQL as string

I also want to use this variable on my form like this

SQL = "select * from userstable where username ='" & TextBox1.Text & "'"

in above statement it is showing error on SQL

Please guide how to create global variable and how could we call it on any form.

Rgds. Mehram