You should attempt GMAT as it is easier than CAT and more than in CAT your work experience shall be considered through GMAT.

If you ave interest in CAT then apply for both.
Also for preparation you can simply join a short term guidance course or professional coaching in order to crack seat in IIM's and if your can't do the same then make a study plan as per your daily schedule and give a dedicated amount of hours daily.

You can go through various online study videos and lectures for your concept clearance.
Good luck.