The good thing is you have an 18-135 already. That means you can test a broad range of focal ranges on your camera to determine what the field of view will be like before you purchase another lens.
For instance, you could have set the 18-135 lens to 50mm and used it like that for a few days. That would have told you that you really want something wider and you could have tested 35mm and 30mm as well before making a purchasing decision.
Also it has allowed you to determine you need a wider lens than 18mm as well. So now you can seriously consider something like the Tokina 11-16 f/2.8, or similar lenses, for your wide-angle needs.
So because you have the 18-135 you don't need to know all this other stuff in any great detail, everything can be compared to that lens to decide how useful it will be to you, except on the very long end of course (135+). The only thing you need to know is your crop factor is 1.6 so you can translate Full Frame (FF) related discussions to apply to your camera. So if someone says 85mm is perfect for indoor portraits on FF you would do the math: 85/1.6 = 53.125mm, and so you know you need a 50-55mm lens for indoor portraits. Similarly you can multiply 50*1.6=80mm. Now you know that if you are looking at FF related articles on lenses and them mention using something in the 75-85mm range you already have a lens to cover that range.
There are differences in other things, like Depth of Field, but you don't have to worry about that, because 1.6 is close enough to FF that the basic truisims still hold true..... F/4 and below are usually for "isolating" subjects, F/5.6-F/11 are for when you don't care that much about Depth of field either way and are focused on maximum detail and contrast, and F/16-F/32 are for getting maximum depth of field.
