Hi there,

The only way (currently, as we've been promised that this will change) to earn credits & experience is to join PvP Great Battle and/ or Blitzkrieg rooms.

And don't worry about the Super Yamato that you're given, it's not usable in PvP rooms anyway.

How do I join these rooms? Is it possible with stock frigate?

Yeah, the stock frigate should be fine.

When you exit to the big map (Called Battle Channel), click on the big yellow plus in a circle near the top of your screen. Another smaller map will appear. Go to the "Battle Area" which is Area 12 (just above the London Harbour).

There you will see either Purple (great battle) or Green (blitzkrieg) rooms and just click to join. Those are the ones giving exp.