If you're asking this question then I'm gonna make an assumption and guess that there are other pieces of your new business plan that should be set up and worked on first, things like establishing a solid database of buyers, having good and legal contracts to use, market knowledge, industry contacts like an agent, title company, and so on.
There are plenty of good blogs, forums, and articles here on BP that have covered the question you asked. And while they're free of charge, they DO cost something valuable: your time to search for and read.
To give a general answer though, the list service websites you're referring to have databases of individuals (with their mailing addresses) that fit searchable demographics. You can purchase lists of people to market to based on things like Absentee ownership, pre-foreclosure, etc.
Once a list is bought, many people use www.click2mail.com to design and mail postcard marketing material to get their phone ringing. Purchased lists can be easily uploaded to the site, then mailed to your target market.
The list service and postal service websites will usually have a written or video tutorial on how to use them. Do some searching here on BP and on the sites you'll potentially use; you should be able to learn the mechanics of it all in a short time. Of course, there's more to wholesaling than just that...