Hi there,
It's true that it's better to do it on the server-side. You have several options here, the most popular being PHP, ASP, JSP, ColdFusion and CGI. I have been working with the 3 first technolgies (ASP, PHP and JSP) and I feel more comfortable with PHP (even if I find JSP the more elegant of all these languages). The choice pretty much depends on your infrastructure (OS, server,...) and how familiar you are with some technologies (ASP will look familiar to you if you know VB when PHP has a C-like syntax, for instance). Whatever your choice is, the solution is rather easy to implement and there is a lot of documentation available everywhere on the web. In PHP for instance, you just have to call the function mail(), along with some parameters, and it will do the trick.
JavaScript is an ideal complement here if you want to validate user input.
I hope this helps.
Cheers,
Xavier