For Canadian Citizens
Canadian citizens do not need a visa to enter the U.S but you must be sure to obtain an I-94 (admission card) from the U.S. immigration official at your point of entry to the U.S.. If you enter through a land border, you will be charged a small fee (it is currently under $10) to process the I-94 card.
As U.S. immigration inspectors are accustomed to admitting Canadian citizens as visitors, not workers or scholars, it is your responsibility to make sure you are granted the correct immigration status when you are entering the U.S. If you are admitted as a visitor (or tourist) without receiving an I-94 entry card noting your immigration status (e.g., F-1, J-1, H-1B, TN, etc.), you will have return to Canada and enter a second time.