Install very easy with some steps:
Step 1: go to Home page of opencart https://www.opencart.com/index.php?route=common/home, then click Download to get the latest package to install.
Step 2: Read the install.txt inside the package above, you need to copy all files inside folder upload into your php site. Here is opencart.
Step 3: Go to the page and setup the database.
Finally, will see the ecommerce opencart.