It is important that you do well in your classes. Graduate schools want to be sure that you can successfully complete the coursework in the program to which you are applying.
If you did poorly in a particular relevant course, you should explain if any extenuating circumstance, such as an illness, family death, etc. in your personal statement.
You should review your transcript for accuracy before you request that it be sent by your Registrar Office. And you should follow-up with both the School and the Registrar Office before the deadline to make sure it was received by the School.
Applying to Graduate School Booklet
Each program should have a list of required courses. Your coursework should be a mixture of science and non-science courses which will give you breadth in your knowledge base.