Dear tulip 18
Totally endorse chscag's opinion. In respect of "how do I access it"; it can be found in Applications. Open Finder, click on Applications—they are in alphabetical order. Click on Preview. My advice is keep it in the Dock. So, when opened, Right Click on the preview App in the Dock and, from the menu, choose "Keep in Dock". Believe me, you'll use it a lot.
This link takes you to an introduction to some of its uses—Print-to-PDF mysteries revealed | Printers | MacUser | Macworld.
Basically, you can create PDFs, merge them, read imported PDFs and lots more. And it come with OS X.
Enjoy.
Ian