There are a load of good flame brushes available for Photoshop, but you do need to actually think about flames when using them. If you're not using Photoshop then I can't really help.
You want a decent mix of reds, yellows and oranges, with a few layers of white over the top, maybe even a light blue around the edges. Gradient fills on some of them, a lot of them should be low opacity.
It's hard to get them looking realistic though, and it might be easier to try and download some actual completed flame graphics, then just C&P them over.