How can i earn new coins for buy new squads?

[Katana Puffin]

And which one you suggest?

[glass]

Would suggest buying from cheapest to most expensive. so that you optimize for options.

Earn coins by doing the achievements, you can see them from your "change squad" or main menu.

[Cathulhu]

You get more coins by getting achievements.
You should really get the Rusting Hulks, Frozen Titans and Zenith Guard, very fun to play.

[Phoenix7786]

2 of the easiest achievements to get with the default squad is simply beating the first Island and drowning 4 enemies in a single battle. Every one of the Rift Walkers can shove units 1 tile over, and Old Earth has lots of water stages on it. Especially watch for the dam mission and the tidal waves mission.

As others have stated, unlock the cheapest squads first. The Rusting Hulks are amazing and many consider them top-tier. The Zenish Guard (if I am remembering the squad correctly) are a little dicey without the Pilot Abe, since he can negate the 1 damage the tank sustains ramming.