Take all the math and lab science (chem, physics, bio) you can while in high school. In college, major in chemistry and take a lot of courses in math, physics, astronomy, and computer science.

Astrochemistry would be a concentration in a chemistry or physics grad program which involves the astronomy department. Most astronomy grad students majored in physics.

Top 20 Astrochemist Universities of World

NASA
LEIDEN UNIV
Leiden Observ
CALTECH
HARVARD SMITHSONIAN CTR ASTROPHYS
UNIV GRONINGEN
Washington Univ
Max Planck Society
UNIV ARIZONA
UMIST
Ohio State Univ
Observ Paris
JET PROP LAB
CSIC
Univ Calif Berkeley
UNIV COLL LONDON
Univ Texas
UNIV PARIS 11
Univ Manchester
SETI INST