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 details used on those sites.
With one Google login you can manage multiple Google Tag Manage accounts. Usually a single 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, rules and macros 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 Tag Manager on your website. You do not have to worry about the container code snippet placement since all this is handled by the plugin.