There are a handful of reasons you need to season a dutch oven before beginning to cook with it:

• It forms a non-stick surface on the pot.
  
• It makes it simple to clean
• It lengthens the life of the pot.
  

Traditional Dutch Ovens are made of cast iron and often are found new with a shallow wax layer as well as mineral oil lining the pot to protect against rust.
This build up must be cleaned off of the oven before the seasoning procedure begins.
Removing the waxy layer can be done with really hot water, soap and some steel wool.
To begin the seasoning process, you must initially heat the lid and pot.
When it is heated, the the pot and lid are coated with a light layer of lard to the inside surfaces.
Shortening tends to be the most effective.
Then proceed to heat your pot to a temperature of at least two hundred degrees Celsius.
Next, place the Dutch oven upside down on the oven rack, with a tray underneath to trap the melting oil from the shortening.
Let the pot "cook" like that for two hours.
At the end of 2 hours, turn the oven off and allow the Dutch oven to cool where it is.
Once it has cooled off and returned to room temperature, take it through the same fat coating and heating process.
This process should be done three times.
The more times it is fired the sleeker the Dutch oven's surface becomes.
If your oven is tiny or not fit for such activity, you can do the same process over an open flame or grill.
The only caveat to this plan would be to make sure you rotate the pot so that it heated evenly.
This process should also keep the pot from rusting, but if rust does occur in some spots, you can simply scour those parts of the pan and treat with the lard and heating process as needed.