This depends on where the squeaks are.
If they are in the frame, tighten the joints, put a rubber washer between the metals being joined together. If you don't have a washer or can't adjust the bolts then lubricate the joint, normal cooking oil works, just put a little on and then make it squeak to try to get the oil to the squeak.
If it is in the springs, this is a bit harder. the easiest way is to get a board to go on top of them. but some people like the springs and this turns the bed into a non sprung bed. Otherwise find the bit that squeaks and either lubricate or replace.