I am 18 years old, and my period is 9 days late. how likely is it that i am pregnant? or is there something else that could be causing this?

It depends: There are many reasons to have irreglular or missed periods. It will be less likely due to pregnancy if you are using a reliable form of birth control. If you were supposed to get your period 9 days ago, most urine pregnancy tests should work in detecting the pregnancy, especially if you use your first morning urine. A blood test will be the most reliable test, though.