A voltmeter measures the electrical potential difference between two points in an electric circuit.
What is the relationship between voltage and current?
Ohm's law applies to electrical circuits; it states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.
Or simply V = I*R
Note that Ohm's law apply only to ohmic materials i.e. Materials which obey ohm's law.
Not all materials follow this V = I*R relationship.
Ref: http://answers.yahoo.com/question/index?qid=20090420163630AAFGtrK
An ammeter measures amperes or flow of current through the circuit.
First connect the ammeter to the electric circuit. Ammeters usually have screw connectors either at the top or at the back of it. Now, Connect the wires to the positive and negative screws and tighten it. Now you can see the reading.
Reference: http://how-to.ask.com/other/how_to_read_an_ammeter