A bit rusty on the banjo? This video lesson from the BCC will get you going again.
If your banjo hasn't been played for some time it's possible the vellum might need replacing - this is the skin, like the head on a drum, which is stretched over the banjo's circular sound body, and tightened using tensioning nuts. Plastic is a good option these days, as it's more constant and less likely to attract moisture and slacken.
There are various kinds of banjo and many different tunings, depending on what kind of music you want to play. Once you've decided which tuning you're after though, tune your open strings to those notes - refer to a keyboard or use an electronic tuning device for the correct pitches.
If you haven't played for a while, you'll need to get used to plectrums and thimbles again, if you use them - medium strength plectrums are always a good bet. If you play with your fingers though, expect some soreness at first (on both hands).