When I talk to people about the science of genealogy, they tend to respond with comments such as , "I built a tree on Ancestry.com in 20 minutes." I am sorry to break this to you but that tree was probably a copy of another person's tree and you didn't research it correctly. In my family tree, there has been at least 3 phantom women named as my 3rd Great Grandma. It took me looking at all the actual records available and using DNA matches to determine who was the correct woman. Even then, our tree gets push back on her identity and I have to pull out the report I wrote for the Boston University class project and explain how the DNA matches prove it. (Sigh!)
The thing about genealogy that makes it so great, is that anyone can get started and then hooked on it! Those people who start it as a hobby (like me, helping my mom) can see the power of getting things right. Once a bad piece of information is out on the internet, it will live forever! If your family is well researched and you don't have any "blank leaves" you probably don't need a professional genealogist.
However, if like my 3rd great-grandmother you have a brick wall that you can't get around, or you have someone who was adopted of unknown parentage, you might consider hiring someone. A genetic genealogist (like me!) uses DNA results in connection with traditional research. If you are unsure of who or where to hire someone, there are great resources online and I will post the links below.
Most genealogists have a "speciality" area, such as German or French genealogy, African American family research or "just" early American research. (This is harder than you think!)
One important note - anyone calling themselves a "Certified Genealogist" or CG has passed the credential testing of the Board for Certification of Genealogists. I am not a certified genealogist but I have passed the Boston University classes (which are insane - contact me before signing up!) and taken many classes, read genealogy publications and attend conferences.
Here are resources on why and how to hire a genealogist for you to review;
The Association of Professional Genealogists
The Board for Certification of Genealogists
Family Search
If you are in the position of thinking you need a professional researcher, remember to schedule a 30 minute free consultation and we can evaluate your project against our skill set. If we can't help you, we will give you ideas on where to go next.