Hello.

I've got this game yesterday and can't figure out what this blue button to the right does. The one with the crosshair on it. I press it and nothing happens..

Can someone help me out?

P.S. for some reason while I was creating account the game didn't let me to choose my username and now I'm stuck with this one O_o any way to change it?

That's your secondary fire.
It only does something if you have a weapon port selected for it on the ship management screen in-station.

On ships with one port like the EC-89 or Centurion, you probably won't use it.