what is the difference between commerce order table and commerce line item table? which table should i consider for created date for an order in commerce_order table and commerce_line_item table.

I can see created dates in both the tables.
The commerce_order table stores data about an order. The commerce_line_item table stores line items (e.g. products, fees, misc charges) in that order. 

If you need to track when the order was created, reference commerce_order.created. If you need to track when the line item was originally added to an order, reference commerce_line_item.created.