What does hamlet decide to do at the end of this speech?

You are probably referring to Hamlet's soliloquy 'O what a rogue and peasant slave am I,' which is an important monologue in Shakespeare's play.

At the end of this speech, Hamlet decides to actually investigate into his father's murder and find the culprit rather than blindly doing what the ghost is telling him to do.

Previously, the ghost of his father appeared and told him that Claudius killed him and urged Hamlet to kill his uncle as revenge, however, Hamlet is unsure whether the ghost is really his father or some being sent by the devil to tempt him. This is why by the end of the speech, he decides to make sure that Claudius is really the murderer before taking action against him.