Ipsit congrats for your wonderful score of 268 out of 360 in JEE Advance 2018 exam. With this score, you can definitely expect your rank to be around 700-800 in the general category. With this range of rank, you can easily get branches like Mechanical engineering, Electrical engineering etc. but I am sorry to say that to get Computer Science engineering branch your rank should be atleast in top 200. Last year closing rank for CSE in IIT Madras was 177. 

The rank range predicted by me could be entirely wrong and you can get much better rank with this marks so right now you can only wait for the final result and then you can decide which college or which branch you want and you which you can get with your rank.

Feel free to ask if you have any more queries. All the best!!