Adding the form is pretty easy - you will probably do it using SSI or an iframe, or even just leave the form as the sole page so you don't have to add it into anything.
Selecting the form to use is the hard part. There are hundreds available, all with some minor modification or feature. Some better with security, some not. Two that I like: form2mail at [web4future.com...] (free) and "simple contact form" at [karlcore.com...] ($5). I like the flexiblity on the data input more on form2mail, while I like the validation and security more on the "simple contact form."