I support the many other comments about Campbells Biology to start off as well. It's very broad and accessible, it has a good index, it has translated glossaries available in many languages, and the older editions are pretty cheap.

I've heard people recommend Ravens Biology as well, but I have no experience with the book. I have used Ravens Biology of Plants which is quite good.

An alternative to Campbell, is Hickmans Integrated principles of Zoology. It touches up on many of the same topics, but less in depth, and makes up for that with a quite complete overview of the diversity of life, something I find a bit lacking in Campbell. Apart from this it covers points in Ecology, Genetics and Evolution, Basic Histology, Behavioral theory, Metabolism.

A final alternative, the free OpenStax Biology book can be found here. You could do a lot worse. I have only browsed trough it on a couple occasions but it's generally well received. It's hard to beat free if you want to get started right now.

More topic bound recommendations:

Ravens Biology of Plans, Rupperts Invertebrate Zoology, Miller's Living in the Environment (Ecology), Pough's Vertebrate Life (Vertebrate Zoology, very short introduction in paleontology), and Lewin's Cell for Cell Biology, Molecular Biology of the Cell by Albert is better as in more complete, but too hard without background in my opinion.

I would personally stay away from the more philosophically laden books at first, this includes the selfish gene by Dawkins. Not that I don't think it's a good book but there is a lot of disputed stuff in there, and without a thorough basis it's often easy to take away the wrong message. If you feel like ignoring this advice other good authors in the genre include Elliot Sober and David Sloan Wilson.