It depends if you are shooting prop aircraft or jets, what effect you are looking for and what equipment you have.
King_Boru has covered the main points. I'm just going to expand them a bit more.
As a very general guideline try starting with these settings....
I'm going to assume you have a telephoto zoom lens with up to 300mm (otherwise you are going to struggle unless you are just shooting take off and landings).
Use AV mode and set it is maximum setting e.g. 5.6.
You may need to decrease this depending on the lens and also the length you are shooting at. As you get towards max zoom on most lenses you will need to decrease to something like 7.1 to keep the image sharp. If you are going to shoot props swap to TV mode.
Set the ISO to what ever will give you the speed you need. This again will depend on if you have IS, how steady your hands are and how fast the target is moving. For your first go I'd aim for 1/1000s for the jets and maybe 1/250s or 1/320s for the props. Remember you want to pan the camera around the area you will be shooting at to get an idea of the what shutter speed you will get. Pointing it at the ground and setting the ISO then shooting at the sky is not a good way to set the camera up as you will end up using a higher ISO than you need.
Use IS if you are handheld and if you have it.
Use servo mode for AF if you have it.
Try to get in close with the aircraft get it in the centre of the frame and give it a second for the AF to get a good sharp focus before shooting. Be steady and smooth with the camera. Of course when a jet is doing 400mph + this is easier said than done! A case of practice makes perfect.
If you are shooting into the sky you will need to compensate for the fact the background will (generally) be a lot lighter than the aircraft. At a minimum of +0.3 for the light aircraft like a T10 Harrier and up to +1 for the something like a dark Hawk.
If you want to do a panning shot you will need to move into TV mode (same as props) for jets go for 1/500s. Check your ISO to make sure you have a usable apature e.g 5.6 to 8.
Hope that helps and remember to post up what you get!
EDIT: One last thing. Remember to check the camera settings on a regular basis. It's easy to set a compensation and then forget or change to TV mode for a prop and forget to change it back then try to shoot a jet at 1/160s
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