I cannot overemphasize the danger of doing this yourself. I cannot and will not assume responsibility for any damage you cause to your instrument by doing this yourself. Read on, at your peril. A critical part of banjo setup is acheiving a good comfortable action. In order to have a proper action on your banjo, you must adjust the neck to the correct tension for your strings.
Most modern banjos have a truss rod in the neck.
This rod serves to counteract the pull of the strings. The necks of most modern banjos are quite thin, and in many cases would bend so much from the pull of the strings that they would have very high action and would be difficult for most of us to play. On some other instruments, the neck must be removed from the rim of the banjo to access this feature. Usually the neck is adjusted by using a small nutdriver, t-wrench or allen wrench, depending on the instrument you have. By loosening the truss rod, you increase its length, which causes the neck to bend in the direction of the pull of the strings.