As other dr has said you should go for vaccination .
2 doses ,28 days apart.
the vaccine is expensive.
however if you cant take vaccine you can Take prophylactic medicine Famciclovir( Microvir, the trade name ) 250 mg x 3 times a day x 5 days..
the therapy is given 7 days after exposure as you will be mid way in to the incubation period .
Next Steps
chicken pox is highly contagious and a close contact with patient ( house hold, residence in the same house ) can give you infection if you are seronegative.
if the exposure is less than 96 hrs ,preferably with in 72 hrs , you can take VZ Immunoglobin, this will prevent you from developing the illness.
if more than 96 hrs have passed , go for vaccine or medicine as advised above.