Typically traditional quilts have borders and modern quilts do not.  That said, your quilt top should speak to you and tell you what it needs. 

Borders function like frames.  They should be consistent with the style of the quilt and scale to the size of the patchwork or blocks.  Don't make the mistake of adding big bold borders simply because you want to make your quilt bigger!  You want your border to enhance the top--not overwhelm it.

For tips on measuring and sewing straight borders--

http://quilting.about.com/od/straightsewnborders/ss/straightborders.htm

And if you ready to move on up to mitering borders--

http://www.geocities.com/quiltfrenzy/miter.html

And nothing says loving like a pieced border--yes, more work--but the results can be amazing--