GRE

If you are planning to take admission in a German University where medium of teaching is English then you do not require proficiency in German language GRE and TOFEL score will be a sufficient condition to meet eligibility criteria. However, you will have to gain working German language skills to live in Germany which is taught to all international students during induction program.

IELTS is valid only for two years. Take IELTS again. Take GRE with good score, and then apply.

First, you join good institute near you like TIME, IMS, Career Launcher, etc. So that you get a better idea of how to solve Questions Papers, Basics, Tricks, etc.
But just joining an institute is not okay.
You have to work hard, keep practising maths problems, logical reasoning, English grammar and all.
As you said, you are in the 2nd year so first, you have to focus on your graduation that is also important, start preparing for CAT in last year or I would advise you to after B.gain 1-2 years of work experience after B.Tech and then go for MBA as it would be better for your admission and placements.
Best of luck.

Before you study any course related book, read Word Power Made Easy by Norman Lewis. This is one single book, which if read dedicatedly will do wonders with your vocabulary and critical reading skills. Once you complete this, you can go for traditional Barron's and Princeton Review, etc.

Hi Aditi,
Every university and country has different admission requirements. Some countries like the USA require applicants to take the GRE while some do not. International students must also take the IELTS or TOEFL to meet English language requirements. You have to be sure of the country where you want to go and then you have to check the requirements.

Hi Aditi, you may do so. The sooner you prepare the better it is. It will also save you from additional gap years in future which you might require for your test preparation. However be aware about validity. GRE scores are valid for five years while TOEFL scores are valid for 2 years.

Hi Viraj,
You can do MBA after gaining some work experience and after taking GMAT and TOEFL. Since your academics are not great you must build a good overall profile. MBS admissions are one of the most competitive.

Do you want one book for all exams? Each of these exams are unique and need different preparation for IELTS prefer Cambridge prescribed book or visit Brirish Council Library. For CAT RS Agarwal, Nishit Sinha Sarvesh Varma, Gautam Puri are few recommended books also try tcyonline and other portal. Manhattan, Kaplan, Argo are few of the recommendation for GRE for other competitive there is a lot of definition of other and there are 100+ competitive exam. Be specific with what you want in life and keep.