Hopefully you have someone in mind before this point, but if it is a spur of the moment urge, find yourself someone you would like to kiss. Make sure they want to be kissed as well, otherwise, the kiss may very well end unpleasantly. You can generally tell if someone wants to be kissed by their body language. If they are willing to be very close to you and touch you, if they have steady eye contact, and if they have been flirting, it is a good bet that they want to be kissed.
I know that this may be a little tricky at this point, but it's very important. By the time you're done here, confidence should not be an issue. Confidence is important because it shows your partner that you want to kiss them and have no doubt. It helps them relax too.