For 3D you use a perspective projection, which makes further away objects smaller. For 2D you just set an orthographic(=parallel) projection instead and everything becomes the same size, also you can pretty much ignore the z coordinate(set it to 0 or just use it for things like having the sprites appear above the background). Thats all.

GLFW - if you like it just use it and dont worry too much about it.

GLFW vs. GLEW - perhaps reread my first posting in this thread. Window librarys like GLFW are for making a window, contex and getting input, so that OpenGL knows where to it should draw. Extension loaders like GLEW just do the tedious task of calling a special OpenGL function which gives function pointers to all other OpenGL functions for you; you just include glew.h instead of gl.h, call its glewInit function after a context is active and the pretty much forget about its existence.

Thank you for the explanation! I'll be on my way to learning OpenGL.