There are several ways how to make a cheap banjo sound better:
Use high-quality strings on your instrument. This often means that you should replace strings that your banjo came with.
Adjust the distance between your strings and the fretboard. They shouldn’t be too high or too low.
Make sure that the neck is always straight by adjusting the truss rod. If you are a beginner, it’s a good idea to entrust this to a professional.
You can improve the sound by changing the banjo’s head if you know that it’s not a good one.
Either adjust or completely change the bridge, as it often provides a huge difference in the tone you get.