Peter wasn’t the brightest student at the school. It is said that Peter wouldn't have been able to defend himself if it ever came to duel and especially if the duel took place with Sirius Black. This is due the fact that Peter was actually less than average when it came to school grades. 

His animagus form of a rat did allow the marauders to immobilize the whomping willow in order to visit Remus in his werewolf form, but even then was he the weakest of the four. It takes a great deal of magic to become an animagus, and he relied heavily on his friends to help him out. Because of his size, he had to be carried around while the other three boys roamed the castle grounds or he would be left behind. 

First Wizarding War

In the first Wizarding War, Peter was one of the members of the Order of the Phoenix. However, when Voldemort became more powerful Peter felt that the Order was losing ground. So, he did what he always had done, which was choose the side with the strongest player. In this case, he became a traitor to the Order and joined the Death Eaters. 

He soon began to work as a double agent for the Death Eaters providing them with essential information. Despite the fact he was always seen as a low ranked member of Voldemort’s army (due to his cowardice and lack of skills) he did receive the Dark Mark and was accepted into the ranks of the Death Eaters. 

During the time that Peter worked for Voldemort, a prophecy about the downfall of Voldemort was spread. Many within the Order believed that this prophecy was about the son of Lily Evans and James Potter, Harry Potter. In order to defend themselves the Potters went into hiding. In an attempt to misdirect Voldemort and a stroke of luck for Peter, Sirius suggested Peter became Secret Keeper for James and Lily instead of himself. As we know now this was a huge mistake. Peter betrayed James and Lily and on Halloween night in 1981, Voldemort attacked the Potters, killing James and Lily.

Strangely, Voldemort was defeated by Harry Potter when his killing curse bounce off the love-protected child and rebounded upon Voldemort. This did not kill the Dark Lord, but weakened him to the point where many believed he was dead. Upon arriving at James and Lily's once beautiful house, Pettigrew noticed what had happened. Peter took Voldemort's wand, and fled in fear that the Ministry would discover his treason. 

After the death of the Potters, Sirius was the only one who knew what really had happened. It was his idea to make the switch and only he, Peter, and the Potters knew it. Furious, Sirius pursued Peter, only to end in disaster. Though Peter didn’t have great magical abilities, he did excel at fleeing. When Sirius discovered Pettigrew, Peter began to scream that Sirius had killed the Potters and transformed into his rat self. Just before transforming, he used a blasting curse to blow the street up. In this explosion, 12 Muggles ended up dying. Peter then faked his own death by cutting off his finger. Sirius was arrested and sent to Azkaban for the crimes Peter Pettigrew committed.

Peter was (unjustly) posthumously awarded the order of Merlin for his confrontation with Sirius, as many believed that Sirius was the Death Eater and that it was Peter who defined him, while we know it was the other way around. 

Between Wizarding Wars

Seeing as it wasn’t safe for Peter to show himself since everyone thought he was dead, he decided to live life in his rat form. Peter feared what the remaining Death Eaters might do to him. It was because of him that Voldemort went to the Potters and ultimately met his downfall. He believed that they saw him as a traitor. 

Peter now needed a new home. He needed a home where he would be able to hear wizarding news, so the logical place was a wizarding family. He eventually found a family that would accept a rat as a pet. This family was the Weasleys. Peter became the pet rat of Percy Weasley and later on he was passed down to the youngest brother, Ron Weasley. 

Peter was ecstatic when Ron became best friends with Harry Potter. It was in that moment he knew he had a redeeming plan if the Dark Lord ever rose again. He had direct contact with Harry and knew he could bring the boy to Voldemort if the time ever happened. Then no one could accuse him of being a traitor.

In 1993, Sirius Black escaped Azkaban, after seeing a picture of Peter. Well, it wasn't exactly a picture of Peter. It was a picture of Peter in his animagus form, but Sirius had seen it enough times to know it was him. In the spring of 1994, the truth about Peter was discovered by Remus, Harry, Ron and Hermione Granger. Sirius and Remus cornered Peter; they decided to kill him for his treason. However, Harry stopped the two friends from committing the murder. Instead, Harry suggested that Peter be sent to Azkaban, in order to clear Sirius' name. This would not happen, as Peter was able to flee again when Lupin transformed. Peter then did the only thing he could, he began his search for his former master. What Peter didn't know was that he now had a life debt to Harry Potter, which would eventually lead to his own demise. 

Second Wizarding War

In the second war, Peter was responsible for many events. He was directly involved in the death of Bertha Jorkins and Cedric Diggory, the escape of Barty Crouch Jr., and of course the rise of the Dark Lord. 

Peter went to Albania to find the Dark Lord, as this was the last place he was rumored to be. In the forbidden forest Peter was left the task of feeding Voldemort milk that he received from Voldemort's pet snake. Once Voldemort was healthy enough to leave, Peter was given the task of helping Barty Crouch Jr. kidnap Auror Alastor Moody. By successfully completely this mission Barty Jr. could take Moody’s place and thus become friends with Harry Potter and sabotage the Triwizard cup from within. During the Triwizard Tournament, Peter helped Voldemort get his body back.

As part of the resurrection spell with Voldemort, Peter was required to sacrifice his right hand. After his body was restored, Voldemort replaced the one he sacrificed with a silver one. This metal hand was strong and impervious to many spells. Unknown to Peter, Voldemort had given him a hand that would stop any traitorous behavior.

Pettigrew was later assigned to "assist" Severus Snape, but Snape treated Pettigrew as a servant. 

In the summer of 1997, Peter was staying at the Malfoy Manor where he was working as a janitor. About a month before Easter of 1998, at the height of the second war, Harry, Ron and Hermione were brought to the manor, Peter watched as Hermione was tortured until there was a noise heard from the basement. When the Death Eaters heard the noise downstairs, Peter was sent down to investigate. 

The noise they had heard was Dobby. When Peter opened the door, he was tackled by Ron and Harry. Peter began to strangle Harry, but when Harry reminded him that he owed him a life debt, Pettigrew hesitated. In this brief moment of mercy his silver hand turned on him and strangled Peter to his death. 

I Solemnly Swear That I Am Up To No Good

Sites:

http://harrypotter.wikia.com/wiki/Peter_Pettigrew

https://pottermore.fandom.com/wiki/Peter_Pettigrew