Welcome to Mobileread!
Its certainly easy to transfer books from your computer to your Kindle. Of course, some people have different definitions of easy
Someone will probably point out a guide for this, but here are some thoughts.
You can connect your Kindle to your PC using the USB cable that came with it. Once you do that you can go to your computer's file manager. e.g. Start -> Computer on Windows XP, and you will see an icon/entry for your Kindle (e.g. "Kindle E:"). Click on that - you are now looking at what is on your Kindle. Click on the entry "documents". You are now looking at all the books on your Kindle.
To copy a book to the Kindle, open another File Manager window and navigate to where you have your books on your computer. Select the book on your computer and then drag it to the "documents" folder you opened on the Kindle.
Now using the File manager window containing the "documents" folder, click on the "Computer" entry in "Computer->Kindle->documents" at the top or on the left. Then right click on "Kindle" and select Safely Remove.
You can now disconnect the cable to your Kindle and see the books you loaded (it might take a few seconds for the Kindle to refresh and show your books).
If that didn't make any sense, post again and we'll back up
There is one catch. The Kindle can only read books of the type Mobi, Text, or PDF. What type of books do you want to transfer to your Kindle? I mean - what format are they in - Epub, Mobi, Txt, PDF? You can tell usually by looking at the file extension (the last few characters) of the file you have on your PC: Mobi is usually ".mbp" or ".azw". The Kindle can read Mobi, PDF, and Txt, but not Epub. If you want to use Epub, or have a lot of books, then we'll need to talk about the Calibre software.