If he or she had sent you a short email introduction and you are ready to respond then make it short as well.
Try to make the email personal and use the first name instead of being formal like you are talking to your supervisor at work.
Don't expect too much hope even if you have common interests because you are still a stranger to him or her so just relax and enjoy the time while your profile is in this dating site.
I just want you to know that this is my first experience with online dating and I'm happy that I joined because I was able to see your amazing profile.
We have all been in the situation where we need to send an email to someone that we have never met.