This is something I learned in middle of junior year and I wish I knew about it sooner, but sign up and attend as many classes in your first week or two. There is no guarantee what is an easy breadth class, as it depends on multiple factors such as your interest in the subject, the instructor’s teaching style and leniency, the format/pace of the class, etc.
Almost all Berkeley classes qualify for one of the breadth requirements, so the most informative decision is attend the ones that sound interesting to you, and further filter out after attending a few classes and/or first assignment. I remember spending lots of time on course catalog and checking out the grade distribution by instructor, and it was well worth the investment.
You can repeat this approach for the math and sciences classes. If things go really well, then you could be doing perfectly fine on CS61A and Math 1B and capable of handling another course such as Data 8 or whatever other course for CS major declaration. Conversely, the CS61A and/or Math 1B may be seriously disappointing whether it’s the instructor’s teaching style or the obscene course load and it would be good to have a back-up class that is more enjoyable. Any class that meets the requirement would be a candidate, so think broadly and assume anything could happen.
After you decide on your courses, make some good study buddies. Some of the problem sets may get overwhelming and often times office hours aren’t enough to answer all your questions.
One disaster scenario you want to avoid at all cost is indefinitely falling behind a class after you didn’t understand a lecture or two and by the time you attempt to cram everything the week/night before the exam, it is too little, too late. Having a study buddy is great as it keeps each other disciplined and not fall behind, and explaining a concept to someone else is the best way to learn yourself too.