First you need a premium Minecraft account,

After that you need to run Minecraft.

Then You have to login -unless you are aready logged in-.

after you login you will need to push the button that says play.

Then you will see a Single Player button and you will see a Multi-Player button.

Click the Multi-Player button.

Then you will see a few buttons either click the Direct connect or the add server put woms ip If you would like or another one.

Then if you picked Direct connect put in the server address to the server in the box, then push the join server button.

 if you picked Add server It will say Minecraft server you can change that to what ever you want then in the second box you put in the servers address then push the Done button then click on then click the join button.

There ya go! now you know how to join multi-player.