When journalizing transactions how do you determine what needs to be credited and what needs to be debited?

Partly rules, partly experience. There isn't any one easy answer.

First you need to make sure you know the rules:
Assets: debits increase, credits decrease
Liabilities: debits decrease, credits increase
Equity: debits decrease, credits increase
Revenue: debits decrease, credits increase
Expense: debits increase, credits decrease

It's the "increase" side you need to memorize. That is the "normal balance," meaning the side where the balance should be, and the side you will increase the account on. Always use the opposite side if you want to decrease the account balance.

So when doing a transaction, you first have to determine what they're saying and come up with the two (or more) accounts to be used, and know whether you want to increase or decrease the balance in each of those account. Then classify them into one of the five categories. Meaning you also have to know what kind of accounts they are. From there, follow the rules above.

However, the interpretation part is sometimes the hardest. And we can't really help with that without having some specific examples of ones that are bugging you.

The rest is practice and experience.