How to add auto number and date automatically when new row populated

I am very much new to excel and vba coding..i'm trying to create a personal expense tracker. I need some help to make it work.

1. I want to add auto number in L column sheet2
2. i want to add default date time when a row inserted through input sheet in M column sheet2
3. Data validations like with out entering into Description it can't move to next input

waiting for your valuable replies..