convergence??????

Please help me!!

I have just purchased a second hand Toshiba rear projection TV (don't know the model number but Iknow it a few years old) and there is a red out line on every thing on the right hand side of the screen. I have been advised to set the convergence but as it was second hand i have no instructions for it and I have no idea what it is. (don't laugh at me). Could anyone please tell me how to do this and what are the correct settings in very simple trems please???

Thank you in advance.