Should I get a mammogram at 18 years old, my grandmother had breast cancer, should I take action now?

No mammogram yet. First of all, the probability of a teenager getting breast cancer is less than one in a million, so there is no need for a mammogram(until age 40). If your grandmother is the only member of your family with breast or ovarian cancer, chances are that you have no increased risk for breast cancer as compared to any other woman in the US. Nevertheless, you are never too young to learn self-examination.
Relax. A baseline mammogram at 40 is appropriate, although in 2011 certain groups stated that it should be at 50. Debate continues. In younger women breast ultrasound is the modality of choice if there is a concern. Remember to do a monthly self breast exam, this way you get to know your breast and can detect changes early.When we think of hereditary breast cancer the person is diagnosed prior to age 50.
Age for mammogram. Since your grandmother is a second degree relative (mother is first degree), your risk is not higher than the general population. It's a good idea to start monthly breast self exam, so that you become familiar with your own breasts. Routine screening mammogram starts at age 40.
No, with that family history you are at average risk, and don't need to begin screening until age 40. At your age, your risk of breast cancer is so low that the substantially higher false positive rate of either mammo or ultrasound means that screening doesn't make sense.