If you're at the Try/Buy screen then you haven't actually installed the application yet. That's one of the first dialogs of the installer. You'll need to run all the way through the installer before you can use the program.
As for using Photoshop, it's a pretty complicated piece of software. You can actually get the equivalent of a masters in it and still not know everything it can do. I would recommend looking for basic videos on YouTube or buying a HowTo book to help you get started.