If You want to Do B.Com First then try to get in to the top Commerce college & U had good score in +2 so definately you will get it..You can choose Xaviors,SRCC,Narsee moonji,BHU for B.COM.and you can start preparation for CAT from your 2nd year so u ll get enough time to prepare for CAT..Because You are graduated from Commerce background you will get benifit of field diversity in IIMs..But you have to prepare for 97+ percentile in CAT along with Good score(80+) in B.com