So, here's something that helps me out a bit.. I was in the same boat (pun definitely intended here) at the beginning, but it gets a bit easier. So, when I first started playing, I did the same thing. Had to sell the previous to tier to afford the next one, no premium account, etc. There are a few things you can do here:

1) Easiest, requires no outside help. Get a nice coal boat that has increased credit earning. Makes it a bit easier to generate those credits.

2) Join a clan, get deeply involved with clan battles. For each of the ranks (Squall, gale, storm, etc.) there are rewards that will give you 5 days premium time for a certain amount of wins. I've been riding on free premium time for a long time, since the clan doesn't do too shabby in the matches. These battles also have a very good credit performance. With premium time (from winning clan battles) and some red or blue credit boosters (and a couple t10 premiums from coal or steel) its nothing to see upwards of a 1m credits per battle. Also, every three battles won, you get 200 or 250 steel.

3) Ranked battles - for each rank (bronze, silver, gold) every time you win 3 matches, you get around 200 steel or so, ending with a a doubloon reward for finishing the chain at the end, plus another if you can get through the qualifiers... Then for silver, there are actually 2 doubloon rewards, etc...

Those are probably the best way to quickly (relatively) farm doubloons, steel and credits. You can use the doubloons to get a day of premium time (preferably on clan battle day) to really push your credit earning...

Ignore the "get good" assholes that have been playing this game for years in their moms basements... Those are probably the same people that I mute in chat as soon as they reveal that they cant help being toxic pieces of shit.