I should know and keep ready:
My general information
My name, address, date of birth, fathers name, Aadhar number (only non residents are exempted from Aadhar), my mobile and email id
My tax information
The assessment year I am filing my tax return for, my Permanent Account Number, My residential status (whether I am ordinarily resident, non ordinarily resident or non resident). Just to keep distinction easy, if I am staying India for long (leave out a small trip abroad), I am ordinarily resident. So I should fill this.
If I was primarily abroad in last 7-10 years doing some job, profession or for some studies or any other reason, and then are now settled back to India, I might be not ordinarily resident, so I need to check. This is not the forum for such a person, you may consult a tax professional.
Similarly if you are permanently abroad, but sometimes visit India and happen to have Indian income that you require a tax return to be filed, you may be non-resident. Again, for this, you may consult a professional.
(Me being the advanced level)
If I am a director, I should keep ready my director information. If I own unlisted shares, I should have complete details of those.