I got 8.8 gpa in 10th,75% in inter,68%indegree,will i get admission in a best business schoolor collegesor iits or nits by having 80+in cat

You have some good Academics, Although for Targeting NITs minimum of 85 percentile is must, For IITs its 90 percentile. But neither of them are recommended except IIT Bombay. Coming to top 5IIMs, XLRI, ISB, FMS, IIFT, SpJain you would need minimum 99 percentile to receive call for further rounds. Hence, it is advised to target 99 percentile to be in safe side if your priority is top notch premier Colleges. For top 25 collges check below for reference. https://bschool.careers360.com/colleges/ranking/2016

You have an average academic records and to get into top premier b-schools like IIM A,B,C,L,K,I, FMS, XLRI and many other private colleges you need to score well above 99 percentile. Even to get in new IIMs you need to get atleast 95+ percentile.

For IITs if you mange to get above 90 percentile then you have a good chance of getting a call for interview except IIT Bombay which again require 97+percentile  and in order to have a high probability of getting selected you should aim for 95 percentile. And of course you can get a call from NITs with 80 percentile but I will not recommend you to go to NITs as they are not good enough in the MBA field both in terms of studies and placements. Rather than them you can prefer some other private colleges which are better in both ways.

So aim for at least 90 percentile in order to have a good chance of getting in IITs.
All the best!!