So, assuming we're talking about a multi-meter, and not strictly a voltmeter, there's two things you need to know. First, voltage and resistance are measured across the circuit, but current is measured in series with the circuit. There should be a seperate lead jack for amps, and maybe one for milliamps. You take the red lead out of the volts/ohms jack and plug it into one of those. If you have no idea how much the current is going to be, start on amps. The milliamps input has better resolution, but if you pass too much current through it you can fry the meter.
Second thing is, are you measuring AC amps, or DC amps? There should be a selector knob or buttons to choose.
In all cases, amps are measured by breaking the signal path and putting one lead of the meter on one side of the break, and the other lead on the other side, so that the current has to flow through the meter to get back to the point where you broke the circuit. Current is like water flowing through a pipe. Think of it as cutting the pipe and putting a coupler to connect it again, only the coupler is also a flowmeter.