Minecraft wiki entry for Playing_on_servers, associated text: Find a server on a Minecraft server listing website.
Read the descriptions and pick one you like. Find its IP and copy it to the clipboard (Ctrl + C for Windows/Linux and ⌘ Cmd + C for macOS). The IP address is often a number displayed next to the name of it, or a domain.
Start Minecraft, click Multiplayer and click Add Server. There should be two boxes, Server Name and IP Address. The server name does not matter a bit. Call it what you like. Now press Ctrl + V (Windows/Linux) or ⌘ Cmd + V (macOS) in the Server Address box to enter the IP or type it in. Confirm your entries by clicking done.
You will be taken back to the server list. The server should appear on it with green bars on the right.
Click the server, click Join Server. Normally, you will be brought directly into the server. If you are not, there is a list of common errors below to help you.