Depending on the technology used for creating your website, it may consist of files and one or more databases.
To create a full backup of your website you must back up all the files and databases that it consists of.
So the first step is to determine which (if any) databases are used by the website, which you want to back up.
Next, you can proceed with creating a backup of the files and the database(s).
Step 1: Creating a backup of the files
We have dedicated the following article to creating a backup of your files: How do I back up my files manually?
Follow the instructions in this article and when done, continue with step 2.
Step 2: Creating a backup of the databases
In the following article you can find detailed instructions for creating a backup of your database(s): How do I back up (export) my MySQL database?
Repeat the procedure(s) described in the afore-mentioned article for each of the databases used by your website.
Once you have a backup of both your files and your database(s), you have a complete backup of your website, which could be used for restoring the website if lost for any reason.