my problem is HOW to access files via WiFi!
How would YOU access a file on another PC via WiFi? Do you even understand what that means?
You make GET requests using a WiFi or Ethernet shield. The GET request runs the specified script on the specified server, with the specified data.
So, clearly, you need to develop a script that runs on some server that knows how to read "the file", and you need to make a GET request to execute that script. One of the pieces of data you could pass to the script is the name of the file to read. Another could be the record number. It all depends on what the script needs, in order to return to you the data you want.
Put the Arduino away for now. Get the script working, calling it from a web browser. When it works, then get the Arduino back out, and make it call the script.