The key thing to photographing thunder/lightening storms is the following:

1) Use a tripod (keep the camera still). You'll be using long exposures so it's worthwhile investing in one for this kind of shooting.

2) Focus on the Horizon but still try and keep interesting features in the frame (helps get an idea of the size of the storm as well)

3) Use Manual Focus. As you are aware It’s best to use manual focus when photographing lightening. Try and focus on something far away 'on the horizon' as usually that's where you'll see your light show.

4) Long exposure and wide aperture. If your camera can set both these settings. Take the maximum exposure you can without washing out the picture (having it to bright). The action will be far from you so a wide aperture and shallow depth of field won’t be a problem (unless you have some very near objects you’d like to include).

5) ISO sensitivity. You'll need to adjust this based on your aperture and exposure time. Take some test shots before the light shows start and see how they turn out. You'll need to make adjustments once you start photographing the thunder for real but you should have a basic idea of what values you'll be wanting.

6) Stack your photos. If you can keep your camera in a steady spot, shot after shot, you can use stacking software to combine multiple strikes into one image. You’ll often have many unexciting shots with maybe just a single strike or faint action between clouds. These can all be combined to make a spectacular image.

7) Be Patient – This photography takes a long time to get right and to get the perfect shots. Just take your time and enjoy!

Further reading: DPSchool