You will need to file for an IFTA license through the state you are registered in if you are engaged in interstate commerce or crossing into Canadian provinces. The IFTA (International Fuels Tax Agreement) applies to commercial vehicles with a gross vehicle weight (GVW) equal to or greater than 26,000 pounds, as well as all commercial vehicles with three or more axles.
If you are doing business in multiple states or Canadian provinces, you will need to pay applicable fuel taxes for each jurisdiction in which you purchase fuel. This filing is done quarterly and is completed by going through the state that issued the IFTA license.