Don't worry about 'needful'. It isn't a word which you will ever hear in standard English.

For some odd reason, the phrase 'do the needful' has emerged ( or survived?) in Indian English. It is not used elsewhere.

'Necessary' is an important word - 'needful' is not.

'Needful' is extremely outdated, and it is never used by native speakers in modern English. You should forget about it.