Both your computer and the computer to which you want to connect should be connected to the same Wi-Fi network in order for you to access their shared files.  If you're connected to the other computer via an Ethernet cable, you should be able to access the shared files. If your router has different channels (e.g., 2.4 GHz and 5 GHz), it's okay for the computers to use different channels as long as they're using the same network name (SSID). Click the Finder app icon, which resembles a blue face, in your Mac's Dock. Below the "Shared" heading on the left side of the Finder window, click the name of the computer to which you want to connect. This will bring up a blank window.  If you don't see any computer names listed in the "Shared" section, close and re-open Finder. It may take a few minutes for connected computers to show up in Finder. It's a button in the upper-right corner of the Finder window. Doing so opens a login form. Type the other computer's username into the top text field, then type in the password used to log into that computer. It's at the bottom of the window. This will prompt your Mac to display the other computer's shared folders. As long as you enter the correct username and password for the other computer, this will allow you to open any folders you see in the Finder window. You may even be able to add files and folders or edit the existing ones if the other computer allows it.
Make sure you're using the correct network. Open  Finder. Click the computer's name. Click Connect As…. Click Connect when prompted. Enter the name and password for the other computer. Click Connect. Browse the other computer's shared folders.