Creating a shape

Hello, could someone please help me create a rectangular shape with one side being a rounded line (to be a side bar on a page). Basically it is a curved line like a ")" that encloses the edge of a page and filled with color. I have watched several tutorials, tried many many options and even figurted out an approximate shape of what's needed, yet it's a bit crooked and not symmetrical. Seems like it shouldn't be that hard. Thanks so much!