This is an interesting question, and I haven't seen very many attempts to tackle the *theory* of the first romantic kiss. The last time I did this I used a line I'd read somewhere around these forums, I think. The idea is to try to arouse a sense of curiosity in the girl, so I said, "I have a question to ask you, but, ah, you might not like it." Her natural reaction (if she's a normal person) will be to get you to ask it, whatever it may be. After a little while, I relented, and then simply went in and kissed her. Seemed to work well enough at the time.
Of course, a lot depends on how things have been going until then. If you've been getting on well enough with a woman to think that the time is right for a kiss, then ideally you won't need to say anything at all to lead up to the kiss. It depends on the context, of course. If it's the end of a date, then just before you leave her you should just kiss her right out, without saying anything. If you're with a group of people, then you should try to get her alone (since women don't really like kissing in front of other people, at least not the first time). You can simply say, "Let's get out of here," take her hand, and lead her somewhere more quiet. If she's ready to kiss, then she'll follow you.
Now I actually try to think about it, it's difficult to come up with something that will work in any context. But the bottom line is this - I don't think that you need to say anything in particular to prepare her for kissing. She'll find it much better if you just kiss her. It looks a lot more confident, too.
As for how to kill your own nerves, well, that's quite difficult. Drink can help, as long as you don't overdo it. Also you can try a little exercise to loosen yourself up - tense some of your muscles briefly (e.g. by curling your toes, stretching your fingers etc.) and then relax them. That can actually help to put you in a relaxed mood (though it looks odd if you do it too visibly!). Otherwise, all I can say is that you close your eyes and think of England (as they say here in the UK).