In order to get around the fuzziness, you either need to focus at infinity, or preferably, use hyperfocal distance focusing. This will ensure that everything from close foreground objects to infinity are in focus. The distance you focus will depend on the aperture you are using, and the focal length, check out www.dofmaster.com for some explanation and charts you can print off and take with you.
For exposure, I guess it's a little hit-and-miss because you don't know the brightness of the lightning strikes, so it's probably best that you expose to get the surrounding scenery exposed to your liking, or just below, so that any illumination by the lightning will bring it back up. Go for a long exposure time, so that you have a better chance of capturing multiple strikes.
Certainly you should always be using manual exposure and not Av.
Let's say you want to go for 10 seconds:
First, compose the scene.
Next, find the correct aperture. Take test shots at 10 seconds with different apertures, and choose the aperture that exposes the landscape as you want it. A histogram on the camera review is handy here!
Now look up the hyperfocal focusing distance for the aperture and focal length you are going to use.
Using manual focus, TRY to find something roughly at that distance and try to focus on it. I know it will be difficult if it is dark, but do what you can. If you have a distance scale on your lens barrel, this may help. Leave the camera in manual focus.
Now, your camera is ready. You can either use pot luck and take a shot, hoping that the lightning will strike in those 10 seconds, or you can try to press the button when the lightning strikes, but I strongly reccomend the first option!
Let us know how you get on!