A multimeter (also known as a meter) is the first test instrument that we recommend you buy if you're just starting out in electronics. They range from US$20 up. Depending on which one you choose to get it will have a different set of features, but most will at least allow you to measure voltage, current and resistance.
There are almost always at least two possible sockets for the positive (red) probe on the multimeter. One will be for measuring voltage and resistance. The other(s) are for measuring current. It is important to put the probe in the correct socket before using the multimeter.
Voltage measurement is achieved by making sure the probe is in the correct socket, selecting the voltage setting, putting the negative (black) probe on the circuit's ground and the positive probe on the point you want to test. You will get a reading of the voltage present. Some multimeters require that you pre-select a voltage range through a variety of methods to enable it to more accurately provide a reading. In these meters, care should be taken to not measure a large voltage when the meter is expecting a small one as damage to the meter can result.
Resistance is measured when a device is not in a circuit or at least the circuit is not powered up. To prepare the multimeter, select the resistance mode and make sure the positive probe is in the correct socket. In this mode the multimeter puts out a small positive voltage on the positive probe and deduces from the current being drawn, what the resistance must be using Ohm's Law . This mode is sometimes supplemented by a continuity function where the meter beeps when the probes are connected via a very low resistance. This is helpful when testing if two points on a device are connected as expected.
Current measurement is achieved by inserting the multimeter into a running circuit. To prepare the multimeter, make sure the positive probe is in the correct socket (often marked A), The circuit actually needs to be broken and re-routed through the multimeter. Be careful to make sure the multimeter can withstand the current you're going to measure. Most have fuses, but it still pays to be careful since who wants to fool around changing mutlimeter fuses all the time?