All Joomla websites have a ready-to-use login feature. This means your website visitors can register and can log in.
Go to Site>Global Configuration and click the 'System' tab. You'll see the settings for the login as below. If you hover your cursor over the labels, you can see an information box telling you what each of these settings are for.

When a visitor registers at your website, they need to activate their account before they can log on. This is done by clicking the link in the automatic email that is sent to them from the website.