Well CVs tend to earn the most credits per game but i guess you won't be driving one in the nearest future.

AAing in GB (especially in a CA) can give you a decent chunk of credits and XP assuming you kill something...
Around 2000 credit gives very good rewards.

Then there's Blitz where you get less exp but more credits per game.

Dont use hedgehogs.

the most efficient but slower method is to raise crew and then sell it... this way you can get millions credits at once.

selling some good a and in demand sailors (+11 or +12) is a nice income

theres also AAing in gb that give lots of benefit (credit, xp and experts :D )

and the money grinder CV
- at low level you should think of staying at blitz for awhile, blitz give more cred also while your in a cv
- pass 61 since your fps are not that strong to take t3-4s and would not survive to long when use as scout, local is a nice substitute, just kill incoming bombers and scouts and nvm those players who whine "LOCAL NOOB", or you could just let the bombers bomb them XD

Fastest way is reach the minimun requirements to trade and ask for help. It's not hard to find someone that gives you 100k credits & 100k points for survive and can class the sailors correctly and buy first DDs/CLs. Join a fleet if you still are not in one.

Another way is to roll +12 sailors and sell them, you can sell a +12 engy or repair at lvl12 around 500k-1M. Also you have the chance of selling ships you are not using and the last thing i can imagine is to buy sailors with olives in premium trade system and sell them for credits.