Hamlet is often considered to be indecisive because of the contrast between himself and Fortinbras of Norway, himself and Pyrrhus, son of Achilles, and himself and Laertes.
First, in Act 1, Scene 1, Horatio provides some background information about young Fortinbras. His father was killed by Hamlet's father in battle, and some of Norway's land was lost to Denmark. Young Fortinbras, young and angry, "Hath in the skirts of Norway here and there / Sharked up a list of lawless resolutes" to march to Denmark and attack in order to reclaim these lands (lines 109-110). This is, the soldiers believe, the reason they have been charged to keep watch. In other words, young Fortinbras has wasted little time in attempting to avenge his father's death; he's even resorting to dealing with criminals and dishonest men to fight with him because that's all he can get. Contrasted with Hamlet's later behavior and the fact that he has no real plan of action to avenge his own father's death and simply waits for opportunity to fall into his lap, Hamlet seems very indecisive and incapable of action.
When the acting troupe arrives (by luck, not by his plan), Hamlet decides to hire them to perform a play that will essentially tell the story of his father's death. His plan is to observe his uncle's response and then see if the new king's conscience seems to bother him, as this would imply his guilt and pave the way for Hamlet's revenge. Before the play, however, Hamlet remembers a speech he once heard the first player give about Pyrrhus and asks him to recite it (Act 2, Scene 2). The speech tells the story of how Pyrrhus came to Troy at the end of the war to avenge his father, Achilles', death by killing the king of Troy, King Priam. Pyrrhus showed great bravery and filial piety, hiding in the Trojan horse in order to get inside the walls of the city and successfully kill the king who'd killed his father. Again, the story of another son acting with immediacy and determination contrasts with Hamlet's own lack of physical response to his father's charge that Hamlet avenge him.
Finally, after Hamlet has killed Polonius, Polonius' son Laertes rushes home from France to find out why the death of his father has been hushed up. The lack of ceremony or publicity for the too-modest funeral has made him suspicious that his father was the victim of some foul play (which he was because Hamlet killed him by mistake, thinking he was the king). He rounds up a bunch of people to support his plight, and they threaten the castle. King Claudius and he immediately hatch a plan whereby Hamlet can be quickly dispatched (a duel where Laertes will have tipped his sword with poison and Claudius will poison a drink which he plans to offer to Hamlet, in case the sword never makes contact). Laertes acts without honor here, but he nonetheless does it out of loyalty to his father; his decisive action provides the final contrast to Hamlet's lack of action. Hamlet only avenges his father when he, himself, is on the verge of death, having been struck by Laertes' poisoned sword.
Altogether, the contrast of these three decisive, quick-to-action, and loyal sons (even when their actions were ill-advised or somewhat dishonorable) with Hamlet's almost total lack of action makes Hamlet look pretty indecisive indeed.
Indecisive is a bit of myth. Shakespeare goes to some length to show that procrastination or delay is a human trait not necessarily a particular trait of Hamlet alone. So, for example, you have in Act 2 Scene 2, the hyrcanian beast, Pyrrhus, who pauses as he is set to slaughter King Priam. Laertes is determined to kill Claudius to revenge his father. His wrath is calmed by Claudius. Laertes wants to cut Hamlet's throat in the church yard and yet in the graveyard, having gotten his fingers around Hamlet's throat, he does nothing. Rather, like Hamlet, Laertes wants to wait for a more opportune time. Claudius in Act 3 scene 1 has in quick determination set down to send Hamlet to England. Polonius gets him to wait till later. Hamlet actually doesn't leave until half way through Act 4. Perhaps if Polonius had not held things up he would not have gotten himself killed in Gertrude's closet. Laertes is set to depart for France, but stalls to lecture his sister to the point where Polonius reenters to hurry him along. Polonius has news for the king and queen as to the cause of Hamlet's lunacy. Claudius and Gertrude are keen to hear it, but, Polonius tells them to receive the ambassadors back from Norway first. Lucianus, nephew to Gonzago makes his entrance to poison his uncle. He stands there on stage making sinister faces of evil intent. Hamlet who is actually occupying center stage at that point, engaged in senseless banter with Ophelia, then yells at Lucianus, "begin murderer...leave thy damnable faces and begin." Sure, everyone has their reason's for delay and so does Hamlet. The play's focus is much more particular about the gap between having formed the resolution to act and actually carrying out the act itself.