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The biggest one, by far, is "don't be intimidated." If computer science appeals to you, then go for it! Lots of people have gone into the major knowing essentially nothing to start with, and have become quite good programmers. There will likely be people in your class who have been programming their entire lives, but don't worry about it. Along with this, ask lots of questions. Not everyone, even computer science teachers, are good at remembering what is obvious and what is not, and you definitely can get left behind in a class if you don't understand a basic concept. Don't feel bad about asking questions that you think are obvious, though. It is likely that it was not clear to a lot of other students as well, and not everyone is willing to ask questions like that.
If you are able, I do recommend trying to program on your own a bit, during your time in college. The classes are likely to only teach you so much, and personal experience (especially when working on projects that interest you, perhaps by solving a problem you want solved) can help your understanding a lot, as well as providing good resume material. If you find that you want to get started now, codeacademy.com is a great place to gain some initial experience, but I want to stress that it isn't necessary ahead of time.
On the subject of people not remembering what is obvious and what is not, I am trying to think of other things that I take for granted about computer science that could be useful to pass on.
Most computer science majors are fairly general, but the jobs can be a lot more specific. You should pay attention to what interests you, be it mobile apps, Web pages, the inner workings of computer systems, data analysis, embedded chips, and so on. Try out as many things as you can during college and see what attracts you. Of course, some jobs can end up requiring several specific areas, such as if you are the only programmer for a little startup. As a side note, game programming is fun, but be warned that while it can be a great way to learn advanced programming techniques, it is a hard field to get a job in, and a demanding job if you do get it.
Many people who are computer science majors love to program, and will actively choose to do it in their spare time. If this is true for you, too, great! It will help you spend time on those extra projects that I mentioned, and mean that if you get a job in the industry you have a higher chance to be doing something you love. That being said, it is by no means necessary to be an excellent programmer. I know plenty of programmers at Google who, after coding all day, leave work and don't think about it again until they come back. So, don't let anyone who says something like "You have to love computer science to be in this major" dissuade you.
Finally, understand that real, industry programming can have a lot less time actually typing out code than you might expect. There is a lot of time debugging, which is trying to figure out why existing code (which you may or may not have written) isn't doing what you expect it to. This can be incredibly grueling and tedious at times, and is probably what I spend the largest fraction of my time doing. You also, depending on the company, may spend lots of time planning and designing how your program will work, possibly by trying to pin down how exactly you expect people to use it, which can be harder than you'd expect.
I hope this helps! Please ask if it didn't address what you were looking for, or if you want more details.
100% of 1 Students