The first step is to apply for a Shareasale affiliate membership if you are not already an affiliate in their network. They are a well run affiliate network, with many interesting merchants available.
In the Affiliate Account Manager section, that is the affiliate dashboard page, you'll see a link marked View those with datafeeds. This lets you browse the merchants who have datafeeds. As you browse the merchant list you'll see an icon with 1's and 0's in it, which is the datafeed indicator. Just browse through the list looking for a merchant you're interested, which should be one whose general product range matches your website.
Once you find a likely program, click on the Join program link and your request to join the program will be sent. Perhaps the program you picked automatically approves anyone who wants to participate, or maybe the merchant will have to approve you.
Once you are approved for a program there are two ways to look at the datafeed bearing programs you are associated with. First in this View those with datafeeds list, you'll see a choice marked Show me merchants programs that... , make sure it says "that I am a member of", then click the Refresh Report button. The page will reload showing just the programs you are a member of which have datafeeds. The choices at the top of the page give you other flexibility to browse the programs, and it's worth exploring it.
The other method is to click on the Datafeeds link, under Get Links. This gives you the same report as just described.
In the program list you see in the right-hand column the Datafeed Info. It describes the number of products and offers you to download product info, or to download a ".csv" file. The .csv file is a data file using the "comma-separated-values" format. The file format will be explained elsewhere.
The datafeed info also discusses "FTP access". Arranging for FTP access to the datafeed file helps you to more readily retrieve the files, but it requires special approval from the merchant. FTP, or file transfer protocol, is an ancient method of accessing files over the network. The important thing is you can write a script to access files over FTP, whereas otherwise you must access the data files manually through a web browser. If the merchant is rapidly changing their datafeed content then it will be far more convenient for you to have FTP access.
In any case if you click on the Download .csv link you will be offered the data file. It arrives packaged as a ZIP file, so simply unpack it using your favorite ZIP extraction utility. For example on a Mac simply double click it (in the Finder) and the Mac will automatically extract the contents.
The file which pops out will be named something like 4303.txt. The numbers in the name are the merchant ID number assigned by Shareasale.