Welcome to WikiTree S.
I did not find Barnett Johnson here yet. You can Search for people using the Find menu and scrolling down to Search - then follow the directions.
The best way to get started here so we can help you is for you to start adding your ancestors. First add your parents with as much information as you have. Then add your grandparents and so on.
Since, as you know I am sure, Johnson is a very common surname - you should add siblings, aunts, uncles, and spouses to narrow the search. Locations are very important as are birth and death years.
Good luck in your search, come back here for help any time. Do not get frustrated! Someone will answer your questions and help you learn WikiTree. Someone will show you where to find sources.