Practice. It'll become easier the more you do it.
And contradictory to what others have said, you *can* have an extremely pleasurable kiss just using lips, and if you're going to use your tongue it *does* matter what you do with it. Don't force it down the other person's throat, don't just stick it in their mouth and wiggle it rapidly (seriously, I had an ex do this to me once, it was extremely bizarre and very off-putting), and don't do the "washing-machine" motion with it.