Hi Lisa,
The easiest way I've found is to embed a Marketo form in an iframe and place it on your site: http://community.marketo.com/MarketoArticle?id=kA050000000KzRQCA0
You essentially create a form asset in a program or in design studio, embed into a blank landing page, and then place this landing page containing the form via an iframe onto your non-marketo page.
You will be able to segment leads that have entered the database via this form and assign them to whatever nurturing tracks you want from there.
Hope this helps.
Justin