The Doppler Effect
Explained
in a rather dry way here.
The important thing from the page is the way you can work out the percieved
frequency from the actual frequency and the various speeds. Sign convention
is important here but the best way to double check with logic (object moving
towards listener f goes up, moving away f goes down)
The equations below take a simplistic look at things comes from the c = f λ equation adjusting for the change in speed by adding the motion speed (v) to the absolute wave speed (c). This is unlikley to be in as much detail as you will need but should at least make sense!
f o = (c + v) / λ
c= speed of wave
v= Velocity of motion (will be + if moving towards source)
f o = Observed Frequency of wave
You could swap the frequency and wavelength around in the above equation.
You can play with changing variables here. It bounces around so looks a bit nicer than the one we look at I the lab. (You are only supposed to have one go at this per day without paying but it seemed to work fine for me)