When you ssh into your AWS instance, you're dropped into your home directory, just as if you're running your own server. From the terminal, try typing:
pwd
You'll likely see something like:
/home/ec2-user
Note that this is your home directory and not your application directory. In other words, your Document Root (where the web application starts with /index.php) is likely something like /var/www/html, so try typing:
cd /var/www/html
ls
The "ls" command shows you the contents of this directory. Here is where you'll build your web application.