3 in 1 Questions: Key Binding, Random letter intertation into String, and TextBox Number only check?

Question

he program I am making requires the user to switch to another window (not connected to this application, this is a macro app.) and press "F12" to start/stop the macro. I don't know how I would "bind" this key esspecial since it's not in that form. I also need to insert a random letter into a String(doesn't matter where, it can be random letter, random place). Also I need to know how to check a normal textBox for numbers only (I know about masked textBoxs but dumby me I didn't start using them so I can't go back and change them all...). Thanks abunch, I post a 3 in 1 so i didn't flood the forum with them, haha.'