I've always wondered, I saw that if you were born in the United States you're a US citizen, it's true?
Wikipedia has a good set of articles on nationality law.
Section 1 of the Fourteenth Amendment to the United States Constitution provides that "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside."
There are very few exceptions to the above. If you were born in the United States, you are most likely a US citizen.
From Ghanaian nationality law:
A person is a citizen of Ghana by birth if he/she was born on 7 January 1993 or born after that date in or outside Ghana at the date of his/her birth only if either of his/her parents or one grandparent was or is a citizen of Ghana.
Ghanaian nationality law has had a number of small changes over the years. If you are less than 26 years old, the above applies to you. If your parents were both citizens of Ghana, then you are most likely a Ghanaian citizen.
Many people, for a variety of reasons, have a claim to more than one citizenship. If you are both US and Ghanaian, then you are eligible to get two passports, one from each country. You can choose which passport you use to enter each country. For example, if you wanted to go to Europe, you would use your US passport, because Ghanaian citizens need a visa to enter Europe.
There is a useful question on this site which can help you decide how to use more than one passport to travel: I have two passports/nationalities. How do I use them when I travel?
As others have mentioned in their answers, you are most likely a US citizen. But being a US citizen is one thing, being able to prove it is another. The document that provides presumptive proof of your US citizenship is the birth certificate on record in whichever state you were born in (although this presumption could be overcome with proof you had renounced your US citizenship, proof that one of your parents was a diplomat when you were born, or the like).
Your first challenge is getting an official copy of your birth certificate. If you are in the US and have the kind of identity document Americans typically have, like a driver license, getting the copy is easy. But if you are in Ghana, you may have trouble getting the state to send an offical copy of your birth certifate.
If you are in the US and don't want to leave, the birth certificate and some other ID, like a driver license and social security card, will be all you need. But if you want to travel to internationally, you will need a US passport. The US State Department has a web page about passports.
The details are complex, but the concept is simple: if you want a passport, you have to prove you are the same person named on the birth certificate. As time goes by, the evidence rots; papers get lost, parents die, people who knew you when you were little move away or die, the elementary school you attended throws away it's old records, the pediatrician who took care of you when you were little dies or throws out the records, etc. So if you want a US passport, get it now, before the evidence you need disappears.
But beware of the downside: US citizens are expected to pay taxes on their income around the world. Even if you don't have to pay because your income is low or you get credit for taxes paid to foreign governments, there is a good chance you must file tax returns. Theoretically, you have to file returns and pay taxes whether you ever have any connection to the US or not. But in practice, they probably won't notice you if you live abroad and never apply for a passport.