You can check the book out from the library.
The Giver is protected by copyright laws. As such, you will not be able to read the whole text of the book online for free. You can, however, check the book out from the library. You can also read a free sample of the beginning of the book from ebook sites such as amazon.com. If you are looking for a specific quote from the book, you can actually search the book either on amazon.com or through google books. Just type the phrase you are looking for and you can often read the page.
As litteacher8 said, you can check this book out from your school or community library. Additionally, you could buy the ebook from places such as the amazon store or the kindle store. There are however some online sites that offer a copy of the book for download or online reading. I have included such sites in the sources.