You must first have a job offer from an employer. You cannot apply for TN status without a job offer.
The TN visa was created under the North American Free Trade Agreement between Canada, U.S. and Mexico. Under this visa, a Canadian citizen may work in a professional occupation in the U.S. if:
1) He/she qualifies under the "Professionals Under the North American Free Trade Agreement" list;
2) the Canadian citizen possesses the specific criteria for that profession;
3) the employment position in the U.S. requires someone in that professional capacity and
4) the Canadian citizen is going to work for a U.S. employer.
Advantages of TN visas
There are several advantages to applying for a TN visa. First, TN visas are valid for three years, which means that the TN visa is valid for the same duration as an H-1B visa.
Second, TN visas are easier to apply for, and to have approved, than other work visas such as the H-1B visa. There are two ways to apply for a TN visa: 1) in person at a port of entry (POE) to the United States; or 2) by submitting an application to the USCIS while in the United States.
Should I apply at POE or through USCIS?
Applying at a POE has several advantages. First, the application fee is considerably lower than applying through USCIS. Second, a decision to grant or to deny the visa is made immediately. On the other hand, applying for a TN visa through the USCIS may take a couple of months. If you are denied a TN visa at the POE, you learn the reason for denial right away and you can attempt to correct it immediately. If your TN visa is denied by the USCIS, it takes a lot longer - and a lot more money - to obtain a favorable decision. Worse yet, you may be required to leave the United States during this time.
Other disadvantages of TN visas
One disadvantage to a TN visa is that it applies to a narrow group of professions, making many Canadians ineligible. In fact, you must qualify within a profession on the TN professional occupations list in order to be eligible for a TN visa. Most of these professions require an advanced degree, or at least a bachelor's degree in a particular area. Some of these professions include: architect, accountant, engineer, graphic designer, lawyer, doctor, management consultant, dentist, pharmacist, and college teacher.
Another disadvantage is that the Canadian citizen must demonstrate an intention to return to his/her home country upon the expiration of the TN visa. This is not usually an issue when it is an applicant's first TN visa. But how does one demonstrate an intent to return to Canada if he/she has been in the U.S. for nine years on a TN visa and is seeking another three year TN visa? The best way is to provide the documents showing that the applicant continues to maintain ties to Canada. This may be done in the form of bank account statements, a provincial driver's license, ownership of real estate, or having a spouse, children or other family living in Canada. However, every CBP agent is different - with different moods and personalities. One agent may deny a fourth TN visa while another may think nothing of it and approve you. Always be prepared for a denial by having supporting documents to prove your intent to return to Canada.
An additional disadvantage to a TN visa is that it is more difficult to apply for an employment-based green card. The reason goes back to the "intent to return" rule. If you apply for a U.S. green card while on a TN visa, this flies directly in the face of your stated intent to return to Canada. There are ways to apply for an employment-based green card while on a TN visa, but it is too complicated to outline in this website.