In order to have access to the Global chat channels, your account must have held a subscription in the past or you purchased a marketplace item in order to remove the F2P account restriction. F2P accounts that have not made a purchase can not access Global1 and Global2 chat.
From the chat menu at the bottom left of the chat window, navigate to the Channels tab, and tick the boxes for the channels you wish to join.
Next, you need to add the channel to the Filters of the chat tab you wish to use. If you don't want to use an existing chat tab, you can create a new one.
In this example I already have a Global chat tab made, and simply wish to add the Filter.
Once the filter is applied, all of the Global1 chat will show up in the custom chat tab. Typing /global1 will allow you to talk in the channel.