I would approach the teacher first. Tell her that you are having issues and ask her for her suggestions. It may be something that she could clear up for you. If she can't, then at least you are making her aware that you are behind and telling her that you are trying to figure out a way to catch up. Teachers like it when students initiate conversations like that.

Then, I would go to your school counselor, advisor, learning specialist, etc. It will be different depending on what kind of school you go to. They should be able to help you make a plan for getting back on track.

One word of advice, don't let it go too long. I've made that mistake and just let myself get farther and farther behind until I was unable to catch up. Act as soon as possible so that doesn't happen. It's not that far into the year. You have time to raise your grades...