If you want to use Facebook advertisement to retarget your website visitors, you are required to install the Facebook custom audience pixel on your website.
Without doing this, Facebook will not be able to capture user-specific events and event-specific custom data on your website. i.e Facebook will not be able to identify people who visit your website.
Once installed on your site, you can start creating Facebook ads that target people who've visited your website.
If you've created one, you can get the code from the link below:
Right there, click Actions > View Pixel Code.
Remember, you're required to paste it between the <head> and </head> tags in your website code.
Keep in mind there is only one Custom Audience pixel per account. However, you can add Standard Events to your pixel to report different conversion events.
If you've not created one for your account, read: how to create a Custom Audience pixel.