It's very simple. Everything is either a debit or a credit. Every transaction has a debit side and a credit side, and this double entry of every transaction keeps the books balanced.
And conventionally we take debits as the asset side, and credits as the liability side. Most of the time you can relate this back to cash, either cash in hand or cash at the bank. If you have money in the bank, then that is obviously an asset, and "Cash" or "Bank" has a debit balance. So that's often a handy start - "where's the cash going here?" and that will help get it the right way round.
So for some sample transactions:
The business sells something and gets cash for it. Cash goes up, so it will be Dr Cash, Cr Sales.
The business buys something and pays in cash. Dr Expenditure, Cr Cash.
New capital is introduced into the business. Dr Cash, Cr Capital.
See - capital is a liability because it is owed to whoever provided it. And it is balanced by the asset of extra cash.
If you're thinking that this must be the wrong way round, because you think of the word "credit" as meaning a plus, and debit being a minus, that's because bank statements are written from the BANK'S point of view. To them, your account is a liability because it's your money, not theirs, and it's balanced by the asset of cash that they are holding.
Revenue does NOT decrease capital. It's a separate page of your accounting book. Where it comes together is Cash.