If you want to customize your SharePoint 2013 site you can use SharePoint designer 2013. You must download the SharePoint designer 2013 from below web link. One good news here is, it is free of cost. You can download and install with out license.
The below is the web link where you can get the SharePoint Designer 2013.
The things is actually SharePoint Designer is not free of cost, had to purchase license. But after SharePoint 2010 released microsoft released designer 2010 free of cost as well when released SharePoint 2013 SPD 2013 also free.
Once open the above link and clicked on the Download link, you will see two versions like 32bit & 64 bit. You can download any version which will be supported to your OS.
If you have any queries, please post a comment below. I will respond you on time permits.