Here is a simple calculator for power, current, voltage and resistance (actually there are 6 different calculators, use the one relevant to the values you know and the values you need to know.

For a better understanding of what all this means, read the articles on the What is Electrical Power and The Dreaded Ohm’s law.

Here is a quick reference with all the possible formula combinations. You may like to print it out and stick it to your multi-meter case or workshop wall for handy future reference.