How to organize your left sidebar to your specific preferences:
Go to the icons in the top left corner of the workspace. They should look like this:
Select the "display" icon, which will pull up a drop-down list of selections for you to choose from to organize your sidebar.
Here, you can select the size of the channel names, organize them by type, send unread messages to the top, and more.
How to hide, leave, or favorite a channel on your left sidebar:
Hover over the name of the channel and select the three dots at the right side, as shown here: