If you are after input and audio as well, then concentrate on SDL. As, openGL focuses only on the graphics side of things.

Like already mentioned, be sure to check out SFML as well, I prefer SFML above SDL but it might be wise to check out both of them.
OpenGL is much harder to use than SFML, even a beginner in programming should be able to draw something on the screen with OpenGL but it will simply take more time and effort. An API like SFML is easy to use and contains functions and classes to easily open a window, draw some pictures, handle user input, play audio and much more. In the beginning I think you should just focus on programming some application or game and not on how to set up an API to do some basic drawing. My suggestion is to play around with SFML a little while. You won't need a book on the subject, just go to http://www.sfml-dev.org, download the 1.6 version and install (check out the tutorials if you don't know how to do this), than check out the tutorials on how to open a window and have some fun creating your first game. Also check out the documentation, where you have an overview of all classes.

I pretty much agree with everything that has been said.
However, I would very much suggest that if you are going to get SFML, that you grab the latest snapshot of 2.0 rather than the 1.6 version as a lot of stuff has been improved since the release of 1.6.
Also, while doing basic drawing and such with SFML doesn't really require much knowledge of what is actually being done through OpenGL, I would definitely say that learning about OpenGL first or while you learn SFML could be a great way to learn about interacting with the GPU at a pretty low level, and as far as I know, you are free to execute OpenGL calls in SFML if you would feel the need to for whatever reason.

You never specified what your main motivation to learn these things is, and I think that this should perhaps be the most important factor for your decision.
If you are genuinely interested in learning more about computer graphics, then I would suggest going with OpenGL or DirectX (depending on personal preference, just try both and see which one you like more), but if your main goal is to make a game (especially a 2D one) then I would definitely suggest going with SFML.

Have fun!

I'd suggest not wasting any time on SDL. SDL is outdated, and lacking many features that SFML provides.

Not true. The latest version (SDL 1.3) is up to date. Check out the docs here. SDL 1.3 uses OpenGL for its rendering backend, supports multiple windows and so on.

SDL has fewer high level features than SFML does (SDL does have addon libraries for some of these features though). For example SFML has image loading built in, but in SDL you would usually use something like SDL_image. SFML does provide a lot more rendering primitives than SDL does (eg, shaders) - SDL definitely aims to be lightweight and low level (eg a thin layer to handle windowing and input while you use OpenGL directly).
SDL has a C API instead of an object oriented C++ API like SFML has. SDL supports more platforms. SFML has networking support built in, SDL does not (but again, theres an addon library for it). SDL 1.3 documentation is still very much a work in progress though, while SFML has solid documentation. Besides that, they are more or less equal.

If you go with the old SDL 1.2, it has a lot of documentation available too and is rock solid, but you lose out on a lot of features like hardware accelerated rendering (unless you manually use OpenGL) and multiple windows.

Having said all that, I would suggest using SFML since it will get you started quicker. SDL would be a good choice for someone who already knows what they are doing and want a base to build a custom engine on top of, but SFML would be, IMHO, a better choice to get started quickly and learn game programming since it provides many more high level features, but these may get in the way if you are trying to build a truly custom engine (which, unless you're an expert, you shouldn't be doing anyway).