There are many ways to do this - it really depends on how you identify who a new customer is in your database. If you mark new customers with a certain field, set up a smart campaign to have them enter a nurture program when data is changed in that field. You can also add leads to this nurture program one by one in Marketo Sales Insight or creating a custom field in your CRM.
Hi Casey, another thing that we use are segmentations to ensure that people can only receive the correct nurture programs at the right time. We have a gatekeeper set up above all of our programs that looks at which segmentation a record is in before they can enter any of the nurture programs. Any changes made to their segmentation will also trigger a re-evaluation of their nurture status in this gatekeeper.