It certainly doesn't help anything, but I have known people who got DUI's and arrested for assault who went on to become attorneys. As Jack Stated each state is different and some schools may care more than others, but I imagine if you apply to a law school in Colorado or Washington where Pot is legal now it won't be much of an issue. California it is basically legal and you I live in San Francisco and openly see people smoking pot in front of police so I don't think schools in any of those states will mind much as long as YOU DISCLOSE WHAT HAPPENED.
Just stay out of trouble and report your incident fully when you apply. The only way this will really impact you is if you don't disclose, because then you will be in trouble for lying not the original offense.
On top of that you are only 19 and may never end up going to law school as a great deal may change for you during your undergrad years. For now stay out of trouble, get good grades, participate in school activities, then when the time comes take the LSAT. Hopefully you do well enough to get in, but really what you should be most concerned about is getting the most out of your undergrad experience.