This is an important question. Here are some suggestions.
First, if you do not understand something, you need to ask your teacher to help you. Most teachers will be glad to help in any way. Perhaps, what you can do is to meet with the teacher after school to get addition help. Math is a subject that you must keep up with, because the material is cumulative.
Second, if you need more help, then you might have to get a tutor. See if the school offers this service. Often there will be student tutors. If there are none who are available, then you might have to ask your parents to hire a tutor. I know that this seems daunting, but it is important to keep up and learn the material.
Finally, do plenty of examples and show them to your teacher. Ask him or her to go over the steps with you step by step. In time you will get it.