i am a BE student (Telecom, third year). i am thinking of giving CAT. i have a lot of doubts

and i would be grateful if anybody could give me correct guidance.
1. On what basis do i decide the field in which i need to specialize- something that makes use of my engineering backgorund, or something radically different?
2. Are classes necessary? will they affect my studies?
3. in case i dont take up classes, how do i go about preparing for CAT?
4. should i give the exam this year or wait until i get work ex?
please guys, this is urgent!!!!!!!!!!!!!!
thanx anyways.