580 EX II Help!
The first item to buy is Syl Arena's book, "Speedliter's Handbook". It will teach you how to get the most value from your flash.
If you use the flash on-camera you should learn to bounce it off the nearby walls and ceiling. A corner where two walls and the ceiling meet is a great place to bounce the flash.
You are almost always better off getting the flash off of the camera. A TTL cord will allow you to get the flash off the camera but still use it in E-TTL mode. Get the 10m (33') one from Syl Arena.
Buy a set of 1/8th, 1/4, 1/2, and 1 CTO filters. Use these gels to warm up the flash indoors or for sunrise and sunset photos - just don't go overboard and make your subjects look jaundiced.
You can buy a set of Rosco samples. The samples are a bit short and will fit tight against the flash head. This is fine unless you shoot a lot of shots in a row, in which case the flash head will git very hot and the gel can melt onto the flash head.
You are better off to buy full sized sheets and cut them to fit properly.
Cut the gels into strips to fit across the front of the flash head and long enough to bow them out about 1/4" from the front of the flash. Put strips of the soft loop parts of some high quality peal-and-stick Velcro on the sides of the flash head. Put small patches of the hook part of high quality peal-and-stick Velcro on the ends of the gel strips to hold them on the flash head.
If you want to use the flash for things like a hair light then look at this set of accessories from ExpoImiaging. It will give you a grid to turn your flash into a spot light. There are also a few gels in different colors. These gels are good for changing a gray cloth or paper portrait background into a colored one.
Don't bother buying junk like the Gary Fong Lightsphere. You can make one from any old translucent white plastic bottle that will fit on the flash head that is just as good, and the cost is exactly zero. They are only useful in a small room with light neutral colored walls in any case.