You need to get at the gland nut which may be exposed or may be under a shroud depending on what sort of tap you have.
The gland nut fits around the tap spindle where the spindle enters the tap body.
Undo the gland nut (there is no need to turn off the water in most cases) and slide it up the spindle, wrap several turns of PTFE tape loosely around the spindle and using a tiny screwdriver or similar, push the PTFE down into the tap body, re tighten the gland nut to compress the tape inside the body.
You may need to do the re-packing in stages to ensure a seal.
.........but if your tap is very old, spindle distorted or corroded you may not get a perfect seal.