The dishcloth pattern N0obKnitter linked to is a good one. It can make a dish/wash cloth or go bigger and make a triangle shawl or a square blanket. It's a good basic pattern to get acquainted with. I'm at that time suck ravelry.com right now looking for a pattern for a hat. They have tons of wash cloth patterns that can be made larger for whatever square or rectangular thing you want. There are scarves too, generally popular. Have a look around and see what catches your fancy, you can ask anything about it or for help here of course!
I should never overestimate my basic arithmetic skills.