The humble contact page is far more powerful than you think. If you don’t have one, you should install one on your site immediately. If you do have one, you should read this article and make sure that you are getting the most out of it.
The Basics
Assuming that your theme doesn’t come with contact form functionality, the first thing you need to do is get a good plugin. We could spend all debating the pros and cons of the multitude of plugins out there, but that seems like a waste of time, doesn’t it? I have two plugins to recommend:
