I have some very bad news for you. This is most likely a scam. The scam usually starts with someone that you meet online, then you develop a "relationship" or friendship, then they ask you to send money for a passport and an airline ticket. Then they will make up something called the Basic Travel Allowance (BTA). This does not exist. They say that in order to leave the country, they must show they have enough money in their bank account before being allowed to travel. Of course, they ask you to send this money and promise to pay you back. And then usually the end of the scam comes when you get all excited about them traveling to see you and on the date that they are supposed to arrive, this person does not show up. You immediately become worried and you try to contact them but nothing you do seems to work. They have just disappeared. Then a few days later, you are contacted by someone that claims to be a friend or a family member of your "friend". They will tell you that your friend was detained by authorities on their way to visit you because they had some unregistered gold or something like that that they were taking as a gift for you. So now the person asks you to send money to either pay a bribe or to pay the taxes on the gold, or something like that in order to get you to send more money.
I know this is not the news that you want to hear and you may not believe me but do a Google search for the terms Ghana, BTA, romance scam, etc.
Here is a link that might help you understand:
On the off chance that this is not a scam, as you already know, there is the K1 fiancee visa option, and there is also the B2 tourist visa. You can look at the link below for all the information that you need on the visitor visa:
Essentially, she must have a valid reason for visiting, must have a way to support herself without working illegally, must have significant ties to her home country, and must have the intent to return home.
A letter of invitation helps with some of this as it will help to show why she is coming to the U.S., how much time she will need and that perhaps she will have a place to stay so that she doesn't have to spend money. Usually you want to show that she is 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. So your letter should state why you are inviting him and that you will provide room and board while she is hear visiting.
Now as to some of the ties, she will need to show that she has significant close family at home, property not easily sold, and a good job for a good company that he has had for awhile that he would not normally abandon.
So if she is young, and without much money or any property or a good job, she would probably be denied. They would see her as 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.