really need to go to a garage for a diagnostic shoud be around £30.
but if your feeling ambitious-
get a multimeter that reads 0hm's
jack up each corner and find the speed sensor in the hub behind the wheel,
trace the wire to the multiplug, un plug it and put the mutlimeter across the 2 terminals and write the reading down. do the same for all 4 sensors (1 for each wheel). then compare the readings if you have a faulty sensor there will either be no reading or it will be alot higher than all the others.
if there all the same you will then have to check the trigger wheels, the fronts are mounted on the outside if the cv joints in the hubs. and the rears ar mounted in side the brake drums. and if thats ok then you definatly need to take it to a garage as it will either be a pump issue or wiring.
hope this helps
sarge