What is double entry accounting?

For the double entry accounting, you should start from the accounting equation which is:

Assets= Liabilities + Capital

For each transaction that the company makes, there should be a debit and an equivalent credit as the equation needs to always hold.

The following accounts are debited when increases:

Assets, Expenses

The following accounts are credited when increased:
Liabilities, Capital,Revenue

For decreases, it is the other way around.