First of all, it would help to know your age. If you never kissed anyone until you were 20 and now you’re 50, that would be a different story than if you never kissed anyone until you were 20 and now you’re 21. So let me know your age.
Second of all, you should never tell someone who hasn’t kissed you that you’re a bad kisser. If you are, they’ll find out. And if they like you, they’ll want to try it some more. You can learn a lot about kissing by being in the moment and feeling what your partner is doing and responding. Bad kissing is non-responsive kissing. Good kissing is being sensitive and responsive and allowing your kisses to reflect your feelings.
So allow your friend to take your friendship to the next level, and when he goes in for the kiss, be in the moment and don’t sell yourself short by assuming you’re a bad kisser or worse, telling him you are before he’s even made his move.