I would like to read ahead before courses and see the roadmap of the course?
Is it cheating if I do this?
Illegal? No. No law exists about this.
You can ask your professors. Have you already registered? The readings can vary, based on the professor. It’s not like high school where all sections of 11th grade U.S. History use the same textbook. So you’d need to know which professors you have.
This is certainly allowed. I also think that if you have the time and energy then it is quite a good idea.
There’s no reason it should be illegal. If for some reason it were illegal then it’d probably be the dumbest thing you’re not allowed to do considering you’re literally helping yourself prepare for the class.
Most schools also have recommended reading lists for incoming freshmen. You could work on that. But once you get your courses and profs assigned you can buy a textbook without facing penalties. Dont write in it until you’re sure you’ll keep the class.
Your college bookstore probably has a link to the required and recommended texts for each class (assuming you know which section/prof). You can then either purchase books directly from them or look online/in your local library to find less expensive copies.
^Yup, the college bookstore should give you an idea as to which textbooks your courses will use. Although, depending on the professor, the requirements may still vary. One of my CS professors said that she basically had to name a required textbook, but that the class would never use it. This kind of thing seems rare though. I’ve totally purchased/rented books in advance to get an idea of what the course is like 
I frequently have students email me personally to ask what books I’m teaching. I see that as a sign of initiative and always let them know.
@zettasyntax not as rare an occurrence as you may think. Sometimes it’s better to wait until the class has started before loading up on the texts. More than once my kid has been told they need the latest edition of a book (brand new, so major $$) or a new book that has an access code, only to be told by the professor the first week of lecture that an older (much cheaper edition) is fine, or that the access code isn’t necessary for getting the online content. Moral of the story: buy used if at all possible (buying through the bookstore is absolutely the most expensive way). Between Amazon and the ease of connecting with other students, buying used is really rather painless and lowers your $$ risk should things “change” between when the syllabus comes out and when you get the skinny from the instructor.
@Undercrackers Yes, I totally agree with using Amazon. I would often just rent my books from there if I knew I had no intention of keeping the book after the class was over. I really just used the bookstore as a way to look up the books I (possibly) needed. The only thing that was annoying was how there would sometimes be things like a USC-version of a Spanish textbook and that essentially made it the case that I had to get the book from their bookstore.