Do you have a nickname you prefer to go by? Or do you want just your first name to show when you post? No sweat.
Click on your name on the upper-right corner of the page from the dashboard to edit your profile and you will see that about half way down your profile there are a few options for you to play with on how your name will show.
If you have a nickname fill that in the “nickname” text box and then when you click the drop down menu next to “Display name publicly as” option, select the name that you want and be sure to save your changes.