First of all, to access Google Tag Manager, you need to have a Google account just as with other Google products. If you are already using Google Analytics or Google AdWords for example, you can use the login credentials used on those sites.
With one Google login you can manage multiple Google Tag Manager accounts. Usually a single Google Tag Manager account is enough for a company since you can create multiple so called containers.
Containers usually equals to websites, most users can use one container for a single website. However it is not needed to create separate containers for separate websites since you can control which tags to include on a single domain. Despite of this in most use cases the easiest can be to handle a separate container for each website.
A container includes all your tags, triggers and variables so that you can control which tag should be shown (aka. “fired”) on which page using what kind of dynamic parameters.
Each container gets its own ID. This is a string like GTM-XXXXX where XXXXX can be any combination of letters and numbers.
If you are using Google Tag Manager for WordPress, you have to enter this container ID only to start using Google Tag Manager on your website. The container code is being placed by this plugin. Because of the special needs of Google Tag Manager, you may need to add a special line of PHP code into your template file but that’s all.
Each container can contain multiple so called workspaces. Most of you are using the free version of Google Tag Manager that allows you to create 3 parallel workspaces. For the paid version (aka Google Tag Manager 360) this is unlimited.
Think of workspaces as branches of your modification inside Google Tag Manager. Lets say you already have your Google Analytics measurement setup and you need to add some Facebook Pixels. Since this can involve lots of changes, you may need more than one day to complete this task. Now lets say you suddenly realize that one of your Google Analytics event tag includes a typo. You change the parameter value but if you decide to publish all changes, you will also publish your newly created Facebook Pixels. But they are not ready to be published!
This is where workspaces can help: if you create a separate workspace for your Facebook tags, you can easily correct any option in your GA tags and publish those changes as they are in a separate workspace. Once your are ready with your Facebook Pixels, you can publish them too and automatically merge your two workspaces into one so that every modification and addition can be live on your site.
You can read more about workspaces in the Google Tag Manager help center.