can I get the train no. 7 to Citi Field at this location of 42nd St. and 7th Ave?

Yes you can. Not the fastest way but definetly the easiest way for someone not from the city.

Yes^^ you just have to follow the signs, kts far from the main entrance

Yes. Train no. 7 pass through this subway station. And I only assume that it will also take you to Citi Field.