Hello aspirant,
After 12th a student earns a bachelors degree.
If the student continues his or her education he or she will earn masters degree.
Since, you are a science student i will explain it you through example of science.
For eg: after 12th i want to do something in science no matter what i will have to get a bachelors degree.
I might choose bsc as my graduate course.
I cannot in any case do msc first and then bsc without having bachelors degree.
I might do bsc after msc only if i want to do bsc again in that field again or in someother field.
All the best.