It might be leaking from the back nut, or it could be leaking between the washer that is under the tap and the sink itself.
Turn off the water and check that with the tap on, no water comes out before you start.
If you think it is leaking from the back nut, try undoing it, then use PTFE tape or Boss White on the threads and retighten the nut and try again.
If it is leaking from the washer between the tap base and the sink, then you might get away with tightening the larger diameter nut directly under the sink - otherwise you need to inspect and replace this washer.
There is a special sprung spanner that makes it much easier to work on the nuts under the sink.
BobG
June 2004