I did very well in HS without trying.
That was my downfall, I expected college to be the same and I failed miserably. Be prepared to actually work for your grades.
I work with freshmen, and I see it happen to a lot of people. In HS people are used to putting in little/no work, do bad on exams and still pass. In college these people do bad on every exam, and some how still expect to pass the class. There usually isn't the buffer of homework to raise your grade.
Time management is a huge part of it. Keep your motivation to show up to class and get to know your professors. Talk to other people and/or check ratemyprofessor to see which professors to take for GE as those classes should generally be the easier of your classes and help boost your GPA.