A DOT number is a Department of Transportation number that is required for commercial vehicles. It does not cost anything to get a DOT number but it must be renewed every 2 years. A farm truck's number is basically one that assists in commercial enterprise of agriculture. Some states don't require you to get a DOT number for farm trucks so check your state requirements.
Gather your information. You should be prepared to answer the questions about if your vehicle is considered a commercial vehicle; whether it will be used to transport cargo or passengers across state lines or out of the country; whether the vehicle will transport hazardous material; if you will always be leasing your truck to another company that has a DOT number; how much the truck weighs; how many people it will carry and several other related business questions.
Fill out the form. Go to the DOT website (see "Resources") and click on "Step-By-Step Registration Guide." The guide will ask you all of the questions referenced in the first step and will determine if you need a DOT number.
Apply. You must submit the application and then make sure you comply with your state's regulations. Each state is different and should be researched carefully. (See "Resources" for a link to all of the state level DOTs.)