What Do I Do If My Guinea Pig Eat Rat Poison?

Take it to the vet immediately!

I would suggest that you say good bye to your pet... And consider watching your next pet a little more closely...  Sorry if I sound a little insensitive:  However, my guess is if you took the time to post this comment...

Instead of rushing your pet to the vet... It is already too late.

Clearly it needs to be immediately taken to a vet where it can be administered medication which make help , such as charcoal.  I would not anticipate a survival however, especially if you have delayed in the least.

Unfortunately guinea pigs cannot vomit like dogs so that is not an option.  There are several types of rodenticide.  The most common is the anti-coagulant--this causes the rat or guinea pig to lose the ability to clot its blood and the animal will bleed out.  The treatment is daily vitamin K for at least 3-4 weeks.  This is potentially a possible treatment for your guinea pig.  
Contact your veterinarian as soon as possible and have your guinea pig examined for other decontamination options.  Decontamination only is possible within hours of ingestion.

Take it to the vet as soon as possible