When kissing, it is very important to remember your hands in relation to your partner's body. When you begin kissing, keep your hands gently touching your partner. Try gingerly stroking their cheek or running your fingers through their hair.
As the kiss progresses, so may your hands. Try running your hand down their back while one hand plays with their hair. Or just embrace them gently. You will find what you are most comfortable with. The important thing is to remember not to let your arms just hang there! Let your hands mirror what your mouth is up to.