First, In order to join a game you have to buy terraria (if you havent yet) Its really easy. All you have to do is go to the steam store and search "terraria". Click on it and buy it. Make sure you have money to. Now just wait for it to install.

First what you want to do is look for Terraria. Then click on it.
Then once you've done that look for this.
Then click play.

once you've pressed play. Nothing happens.
Just spam click play and soon it should start up.

Once terraria is launched up you will see this.
Click on multiplayer. and you should see this.

If you are joining a friend then click "Join via steam". Then something will pop up that has all of your friends on it. Click on the friend that you join and click "Join game"

If you are joining a server then press "Join Via IP". Once you press it select (Or create) Your character. Once thats done this will show up.
Note: If you have joined a server before, Under the "Enter server Ip" box is the server that you joined recently. Its there so you dont have to memorize the IP. just click on the server name and you will connect.
Now to get a server IP you should google "Terraria servers". When you find a server enter the server Address . Thats the examplename.example.example.net part. Not the server port part. Once you correctly enter the server address you will see this.
Most servers have "7777" as the server port. So if it doesent then put in the correct server port.

Now if you did all the steps right you should be in a Terraria server.
Hope you injoyed my guide.