It's pretty simple but you need Adobe Acrobat (not Reader) to do it.
There are various ways to go about it but once Acrobat is installed, most of your applications will have a "convert to PDF" option.
Once you create the PDF, you can open it in Acrobat and apply various security features. Keep in mind that Acrobat security is pretty easy to get around so if anyone really wants to bypass it, they can.
So, once you get Acrobat, you'll want to read up on the manual a bit and if you have any specific questions, ask in the Acrobat forum.
How serious are the security issues in PDF? I want to create my own e-book product, how can I lock it securely?
How serious are the security issues in PDF?
The security features are relatively easy to get around in my opinion. There are various PDF unlockers available.
I want to create my own e-book product, how can I lock it securely?
Most ebook producers use Digital Rights Management solutions. They normally come with a healthy price tag. If you were just going to do a book or two, it wouldn't be worth it. You may be able to use Google to find a service that will lock one or two for you but I haven't looked for one myself.