You can help ease any nervousness the same way you would ease nervousness about a presentation you'd give at work; know your material. And you already know the subject pretty well, since it's none other than you, right? Plus, the person on the other side of the screen is in the same boat, taking a chance on meeting somebody new, and he/she is about to get to meet the wonderful you, so the chances are pretty good that you might be about to meet somebody with lots of great qualities, too.
Keep things light, enjoy some pleasant banter, and just assume that everybody is hoping to make friends initially. If something more develops, that's just a bonus. If you look at it that way, you won't feel pressured or make anyone else feel pressured, either, and you'll both enjoy getting to know each other. Keep up with the current events. But don't get political, at least at first, have a non-offensive joke or two to share, and be willing to share some topical information about your interests, just as you would if you had met in a group of friends. Before you know it, you'll both know if you want to pursue something more, and you'll have gotten over your first-time personal ad jitters without even noticing.
The initial response to someone's ad should be a brief one. You do not have do speak of yourself a lot because a link to your profile automatically appears at the bottom of your response when you reply to someone via e-mal on the web-personals.
Your response should be a response to a certain ad, i.e. you should respond to things that were mentioned in it. If you just copy a pre-fabricated text into the body of your letter, your online correspondent will at once feature that out. For instance, someone mentioned in his/her profile that he/she likes rock music. The person would be pleased to hear some word on the theme from you. Remember, those words should not be a formal reply, it would be clear that you didn't bother to read the letter carefully. It is more honest to write that you are not interested in the subject, that to act as if you are.
When your recipient reads the answer he/she will either agree or disagree with you point of view. In either case he/she will most likely respond and thus bring the situation to a more personal level.
In the very first e-mail you could also ask some questions. For example, your addressee said he/she lived cats. You better ask the person to be more exact about that. What does ?love? mean? He/she may love animals in particular and have one cat. But what if the house is inhabited by ten cats, several birds, two dogs and some exotic animal (an iguana will not be the worse, presupposing that it is a small specimen of the group). That would be ok, if you are not allergic to fur. And what if you are? And find this out only when you visit your sweetheart? With this in mind inquire about what exactly does ?love? mean with regard to anything.
The online dating game is the online waiting game. You may receive an answer not earlier than in a week. Do not worry and do not pester your recipient with letter. If the person is interested he/she will respond, if not ? that isn't your match and there is no reason to be angry or crazy about it. What will certainly not make you more appealing is sending 100 more e-mails. After that number of unreciprocated letters you may get an answer you least of all awaited.