i am 3rd year engineering student i would like to start preparation for CAT so suggest me the best way and any online sites

Initially for the first 5–6 months you should:

1. Start reading voraciously: Op-eds of dailies like Hindu, Economic times, Articles from websites, Non-fiction novels
2. Solve puzzles and improve lateral thinking:
1. Can start off with “Puzzles to Puzzle you” by Shakuntala Devi
2. Sudoku, crosswords
3. Buy a book on vedic mathematics and start off learning how to calculate fast

Post that you can start preparing full throttle! The approach I think will work:

Set weekly targets and aim to finish the same before time. See while studying work may come up suddenly (adhoc work or surprise test, etc.) and I feel u need that flexibility to do well. Hence, juggling between college and MBA preps becomes all the more desirable. Also innovation in preparations also helps a lot.  Set targets like:

a) Half an hour a day of reading
b) two Quant concepts a day (theory)
c) 1 DI set, 1 LR set (preferably from old mock papers)
d) Half an hour a day of quants practise

Follow this for around 7–8 months and you should be be able to finish the concept and topic wise practise part of things.

In the latter prep phase mocks will start pouring in and giving them enough time and attention is of paramount importance. Also one should thoroughly experiment and finalise on a strategy for the actual exam

Keep a track of the following while analysing mocks :

1. Accuracy
2. The best approach to solve a problem
3. Strong-Weak areas

Start slowly with 1 mock per week - taking atleast 3–4 days analyzing them and later on graduate to 2 mocks per week - slowly you will see that the time taken for analyzing also reduces!

Also, as you delve into mocks keep exploring shortcuts, tips and tricks to solve problems in an easier and lucid manner!

All the best!