CAT

CAT is at all India level and CET is if you want to do MBA from any college in Maharashtra. Choose college which you want and then check which exam they accept.

Yes you are eligible if you secure more than 50% in graduation.

Yes you can. As long as you are eligible as per the X,XII marks criteria (average &get;80 for Science, &get;77 for commerce, &get;75 for arts background), you won't be directly ineligible for IIMA.
But since your score is less in graduation, you need to compensate for the same by getting a higher percentile in CAT. Based on CAT 2015, you will require 99.8+ to be on the safe side for a call from IIMA.
All the Best!

Their are many options like you can appear in MAT,XAT,CMAT,ATMA etc. These are some entrance management exams which are accepted by many B-Schools.

Hi,
Scoring more than 95 percentile is not an easy task. The most important part of preparing is practicing. You must devote 3 hrs daily to practice quant, verbal and logical reasoning. Devote more than three hours if you are taking CAT This year itself. This is above and after your coaching class which you might be taking or will take.
Apart from all this, don't underestimate the power of regular tests and feedback. Take mock tests to know where you stand and keep working on your weaknesses. Pay attention to your errors and mistakes and work on improving them.

If you are serious about cracking CAT, don't look for free stuff. Free apps and all won't always be good.

You should start your preparation for CAT in third year.

Which percentages?

90% is required to get admission in IIM Bangalore. Some time it also depends on your luck.

No.
If you are going or executive MBA, then these exams are not needed.