lightning photography is actually REALLY easy! All you need is the guts (or stupidity?) to be one of those people headed outside when everyone else is going for cover! LOL! You have to judge your aperture by how long you are going to keep your shutter open, is there rapid lighting? or like pp said, lots of heat lightning? even street lights will cause too much light! Its easy to blow out your lightning if you keep your shutter open too long with a low fstop. Here are a few examples that I have done recently:
This one was taken near the end of a storm, I knew that my shutter would be open for between 15-25 seconds (25 is usually my max). So I put my apt on f18. I don't bump my iso over 200 because I haven't gotten great results with that, you may be different.
What you need:
Camera
Tripod
remote (optional but suggested)
Exposure 13.7
Aperture f/18.0
Focal Length 18 mm
ISO Speed 200
This one was also a smaller apt even though my exposures were super short (1-3 seconds for most from this storm) But the lightning was REALLY bright, so I didn't want to blow it out. You will know if your lightning is extra bright because the whole sky will light up like daytime! LOL!
Exposure 1.1
Aperture f/16.0
Focal Length 18 mm
ISO Speed 200
This is an example of blowing your photo out because of your apt not matching your environment and shutter speed!
The lightning was MEGA bright this night, and when you factor in that I left my shutter open for almost 15 seconds with an apt of 10.... It just didn't work! So when you learn the rules (ss,apt,environment) and you factor those in its a breeze!

Exposure 14.8
Aperture f/10.0
Focal Length 18.3 mm
ISO Speed 200
So I know this was long but just remember these points:
BEFORE SHOOTING
1. have a sturdy tripod... if yours isn't, hang a bag of sand or rocks from the middle to sturdy it.
2. Camera settings: Manually Focus for the horizon if you can... if you can't, find the farthest thing from you and put that in focus. Set to blub setting. if you have a remote (which is best) make sure your set to shoot with remote)
3. always check your environment, street lights, porch lights, car lights, they will all blow out easily!
4. count how long in between a few strikes, this will help you know what your ss will be and in turn your apt. Long SS = Higher f stops (like 18)
START SHOOTING
1. Open your shutter and wait for the lightning! If you get to about 20-25 seconds with out one, close the shutter and start again. When you see the lightning, wait one second, then close your shutter.
2. Always check after your first strike caught to make sure your results are what you wanted. (check your focus! I hate when I get that wrong!)
3. Enjoy the show, but DO NOT get caught up in it! Its easy to do, but if you find the lightning is too close go inside. (I actually shoot hanging out a window in this case, though I know its not much safer. LOL)
4. Check periodically.. but do not check after every single shot because you WILL miss a great one while your peaking!
Hope this helps!!