Although we would need more information in order to give you an appropriate response, in general, you are subject to various permits to do this in Lebanon or West Lebanon, New Hampshire, and locally from the City of Lebanon.
In the State of New Hampshire you would first need to obtain a license to sell food from Food Services Division of Health and Human Services, and probably a Hawkers and Vendors permit from the State of New Hampshire too. From the city of Lebanon directly, if you want to park your mobile unit in a public place and sell food, there are designated vendor parking spaces, you would need a permit from the City's licensing board. You can contact the licensing board by emailing Paula Maville.
If you are going to sell food from your mobile unit on private property, you would need permission of owner, and the owner of the land would have to get a zoning permit from the city for the mobile unit to park there and sell food. Only certain zoning districts would permit this. I would look into what the State of New Hampshire requires first. Visit the state sanitation page.