A metric tonne is a measure of mass, but a litre is a measure of volume. To make a conversion between the two you will need to know the density of the liquid, meaning how much space it takes compared to its mass. The density of different liquids varies, hence the fact that a can of petrol and a can of water of the same volume do not weigh the same. Most conversions of metric tonnes to litres involve fuels.
Check you are using the right terminology. A metric tonne is the same thing as 1,000 kilograms and has this meaning in all countries. It shouldn't be confused with the United States use of "ton" to refer to 2,000 pounds (807 kilogram), which is known in the UK as a short ton. Nor should it be confused with the old UK use of "ton" to refer to 2,240 pounds (1,106 kilograms.) This potential for confusion is why the specific term "metric tonne" was coined.
Find the conversion factor for the specific fuel you are using. The conversion factor is simply the density expressed as a number you can easily use for conversion calculations. BP lists the following guideline conversion factors: 1.844 for liquid petroleum gas, 1.351 for gasoline, 1.240 for kerosone, 1.192 for gas oil or diesel, and 1.065 for fuel oil. If you are working with a specific batch of fuel, the supplier may be able to provide the actual conversion factor.
Multiply the weight in metric tonnes by the conversion factor. This gives the volume in kilolitres, a more useful measure in most practical situations. If you want to find the result in litres, simply divide the result in kilolitres by 1,000.
Converting metric tonnes to litres is simple if the liquid is water. One metric tonne is a thousand kilograms which is equivalent to a thousand litres. Bear in mind that the precise figure may vary slightly depending on whether the water contains chemicals other than hydrogen and oxygen.
The density of liquids such as petrol can vary with temperature. The BP conversion factors are described as averages only. If you need to know the absolutely precise mass of a real volume of liquid you will need to weigh it rather than use on-paper conversions.