You can help your mother in many ways, and asking this question proves you are willing to do whatever it takes. The first step can sometimes be the hardest step to take; my suggestion is you should talk to her first. Let her know this is hurting you, and you feel that it would be better if she quit. Now, this may not always work; Meth is a high energy drug; therefore, it is harder to stop using. This does not mean that it is impossible, however.
In this case, if talking to your Mom does not work, talk to an adult you trust such as a counselor or a family friend that you feel can help. Another outlet would be to find your local crisis center. It is free, and they will be able to provide someone for you and your mother to talk to.
By taking this step towards helping your mother, you are taking a stand against drugs, and that alone is a sign of a true leader.