1)Party Pooper. it's someone who spoils the party, spoils the fun people are having.
2)To flush something out.-to get the truth out of someone or get something out of the system.
The cops were cross examining the culprit. I am sure they will flush the truth out of the guy.
Marcy overdosed on pain killers and the emergency staff had to flush her stomach out.
3)This cutie really knows how to get jiggy with it.- dancing, slang for sexual intercourse or getting motivated to do something.
4)If you do this you won`t go far wrong.= you can't really go wrong if you do this.
5)Goober that one up.= goof up? I am not familiar with that one.
6)Dead ringer = a carbon copy of someone else or something else. Looking exactly the same " My brother Sam is a dead ringer for Bratt Pitt" - he looks exactly like him.
7)Go on get it off your chest.= it's an idiom: to get something off one's chest = tell me about and you will feel better, vent your feelings that you kept bottled up for too long.
8)He's as straight as a gun barrel.-you can really trust that person to have the integrity, be honest and truthful ( no bs). The context, however, might also indicate a heterosexual orientation.