This may be over your head, but it is vitally important that you think about the type of photography you are interested in. Based on that, you can figure out what lenses you really want and then decide which camera system to invest in. Lenses range from expensive to super expensive and will be the bulk of your expenditures.
I'll give you an example of what I'm talking about. If I was all about portrait photography, there are a number of options, but I would lean towards Canon because in my mind they have the two best portrait lenses out there in the 85mm f/1.2L and 135mm f/2L. Canon would also be a great choice for long telephoto primes like the 300mm f/2.8 or 400mm f/2.8 for birding.
When it comes to zooms, Canon is great, but I would lean more towards Nikon. Their 14-24mm f/2.8 is unmatched. Their 24-70mm f/2.8 is newer than Canon's and might edge it out in image quality (which is saying a lot because the Canon is wonderful). Same goes for the 70-200 f/2.8. Lots of folks think the Nikon offering is better. Nikon also has a 200-400 f/4 zoom that Canon has not built. So like I said, if you wanted a suite of zooms covering the range from 14-400mm, Nikon really has it covered.
Having said all that about Canon and Nikon, it's probably important to consider the size of these lens systems. Both are huge. If you want a lower profile camera and lens system, Olympus has a really nice small camera and a fairly well received group of lenses for their four-thirds mount.