craftgrrl | skirt/dress sewing help please...?

sorry i don't know where my camera is.

I'm making a dress now, out of non stretchy fabric. (I think it's just regular cotton). I pretty much started with two rectangles of fabric [1], and sewed them together. Then I sewed down the sides to make a better silhouette [2]. That may or may not have been a bad idea, but for now the front looks how I'd like it to look [3]. BUT, the back (where my back meets my butt), is just a mess of excess fabric [4].

I have no idea how to properly cut and sew this so the fit is better and not baggy and saggy. I'm guessing that sewing in darts would help, but I honestly have no idea how to do this, and all the websites I'm looking at aren't helping.

can someone help me please??