For computer science, do we start immediately after we’re in college like the first semester or do we have to wait till junior year? I’m applying to colleges such as nyu poly, uMaryland, Purdue, uMichigan, berkeley, etc (popular engineering schools)
You would start taking the introductory level CS and math courses in the first semester.
Depending on the school or division within the school, you may be admitted directly to the major, or admitted as an undeclared student. In the latter case, you would have to declare the major later (possibly being required to complete prerequisites); if the major is at capacity, there may be a threshold GPA or competitive admission process to declare the major.