Summarize the following:
You'll need to know the address of the Minecraft server to which you want to connect before you proceed. You can find free Minecraft servers on sites such as minecraftservers.org. Click the Minecraft app icon, which resembles a grassy block of dirt. If you aren't logged into your Minecraft account, enter your email address and password, then click LOG IN. It's a green button at the bottom of the launcher window. This will start Minecraft. This is in the middle of the main menu. If you're on a Windows computer, you'll need to click Allow access on the Firewall prompt before proceeding. It's in the lower-right corner of the window. Type the name that you want to use for your Minecraft server into the "Server Name" text box. Type your server's address into the "Server Address" text box. It's near the bottom of the window. Once you see the server's name and a status bar appear in the main window, click the server's name to select it. It's in the lower-left side of the window. Doing so will connect you to the server.

Summary:
Find your server's address. Open the Minecraft launcher. Sign in if prompted. Click PLAY. Click Multiplayer. Click Add Server. Enter a server name. Enter the server's address. Click Done. Select the server once it connects. Click Join Server.