I know that I need a fortify magic on other spell or scroll. But I can't find one. where do I get one? Or do I have to create one or something? I'm a bit frustrated with this one.

The scroll you need is in a chest next to Denel, along with the other scrolls. If you want to read more about it, you can do so http://www.uesp.net/wiki/Oblivion:Vahtacen%27s_Secret

Thank you. I should have thought of that.