It's very good that you are asking these questions, being new you've not experienced what "GOING BAD" really is
First NEVER, NEVER EVER leave your patient... scream, call out, hit the emergency button call a code with the patients own phone, but don't leave them..... well ok, you can stick your head out the door and scream for help.
Then the ABC's starting with you....assess yourself, deep breath and check airway, breathing and circulation. Guppy breathing... 5 times a minute or agonal does not count, nor does the non responsive can't maintain their airway and they are a limp doll, it's an emergency.....
So is a thready pulse when you can't get a BP... just compress... the patient WILL OBJECT if you shouldn't be doing it, and they'll object quickly.
After you've called a code, you don't leave, you direct your team to check a blood sugar AND shut off the PCA pump and give narcan if they have a med pump...the usual culprits if patient not septic and surgical.
Order someone to page the MD stat, get you the chart and pull up the am labs.... while the code begins... YOU don't code, you're needed then once the team arrives to talk with the doc, call the family and answer a ton of questions... let the code team then do their work... don't leave!
I"m missing a ton of advice, but time is brain tissue... don't try taking a bp for 10 minutes... if you can't palpate femoral pulses and no bp registers, get on that chest stat...not breathing well... ambu them.... if on a pca, reverse it... and check a blood sugar asap.
You don't have to do it all, shout out those needs until the code team arrives and NEVER leave your patient.
Thats the best advice I can give as and ICU nurse who runs to floor codes and frequently finds an almost dead patient alone, with no interventions having been done when I get there.