In essence, there is no correct way to respond or react after a kiss, in particular a romantic kiss. Kissing is an intimate and personal experience. Typically, your response will be determined by a few aspects. If the kiss is unwanted, you may want to avoid embarrassment and avoid hurting the other's feelings, while at the same time not mislead the other person.
Knowing how to avoid awkwardness and act naturally after a kiss may seem like a very simple concept, but for the young or inexperienced, it may take a bit of practice. Regardless of experience or age, there may be awkward moments, and how you should act depends upon several factors. Your feelings for the person you've kissed should determine your response.
In the case of a casual first date, your reaction will most likely be a positive or negative response. Assuming you are attracted to the person and want to convey those feelings, a simple gesture such as a soft smile or eye contact after a kiss might be all you need to do. Just as important is knowing what not to do. If you enjoyed the kiss, saying something silly or giggling will probably not get the message across.
If you didn't enjoy the kiss and do not want to encourage anything more, try to be as tactful as possible. Your common sense will probably lead you in the correct path, and prevent you from making a brutally honest comment that would deflate his or her ego and cause hard feelings. If you're uninterested, gently step back in a subtle manner.
Other circumstances may cause awkward moments after a kiss. For instance, if two people who have been involved in a platonic relationship for some time, awkwardness can result when one person suddenly makes intimate advances. If you are merely friends, a spontaneous kiss may catch you by surprise. Your feelings, especially if they've changed, will dictate how you may react after kissing a friend.
Maybe your friend's kiss will be intimate and suggestive, yet you are uncertain of your feelings. In this case, it's best to be honest by explaining your uncertainty. You should, however, avoid any drama or awkwardness by keeping it casual and light. There will be time to talk things over later.
Regardless of the nature of a relationship, it's not a good idea to over-think how you will react after a kiss. Don't put the emphasis on yourself or how you are meant to react. Go with the moment, follow your heart and listen to your natural instinct.