Do not focus on the letter. You are supposed to apply yourself for the visa and you can include the letter in your application package or take it to the interview. You can look at the link below for all the information that you need about a visitor visa:
Essentially, you must have a valid reason for visiting, you must have a way to support yourself without working illegally, you must have significant ties to your home country, and you must have the intent to return home.
A letter of invitation helps with some of this as it will help to show why you are coming to the U.S., how much time you will need and that perhaps you will have a place to stay so you don't have to spend money. Usually you want to show that you are going just for a few weeks or so. If it is longer than that, immigration will wonder why and will become suspicious. They are almost sure to deny someone that says they want to visit for 5 or 6 months unless they have very strong evidences of everything else I listed above. But keep in mind that a letter of invitation is a small part of the process and isn't even a requirement (people mistakenly believe it is the most important) and it can sometimes be used against the applicant. For example, if the consular officer believes that the applicant will have too much support meaning that they could stay illegally in the U.S. or if there is a romantic connection. Sometimes it is better to not even have a letter of invitation. The most important part of a case is to have strong ties to the home country.
Now as to some of the ties, you will need to show that you have significant close family at home, property not easily sold, and a good job for a good company that you have had for awhile and that you would not normally abandon.
So if you are young, without much money or any property, you could be denied. You would be a risk for staying illegal. Also, persons that are planning to travel with their spouse and children are a risk as well because some of their significant ties are actually going with them.
I mean some people have been approved with less, but I am trying to give you the idea for the best chance.
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