He was originally the leader of the Senju clan. He stood at the top of the ninja world and everyone acknowledged his superiority in a war-torn world. After fighting against the Uchiha clan for so long, the Senju's main rivals, he eventually offered an alliance with them that they accepted. Soon after they made a deal with the Land of Fire to form Konohagakure, and the system of one country and one ninja village was created and quickly adapted by the other countries, as a result the world became rather peaceful.
He taught the "Will of Fire" to the village and had a special blood-line ability to create trees that he used to create the forest surrounding the village. Hashirama could also control Bijuu and originally had several Bijuu under his command, but he gave them to other countries as a sign of trust and to balance power.
Some time later he and the leader of the Uchiha clan, Madara had a falling out over the rights of leadership. Madara was prideful and paranoid, afraid that the Senju would subjugate the Uchihas if they were in control so he challenged Hashirama to a duel to the death, but Hashirama won and became the 1st Hokage. Their battle however was so great that it shaped what is now known as the "Valley of the End." The village engraved statues of Madara and Hashirama facing each other in the sides of the valley's waterfall. After the battle where he gained control of the Kyuubi from Madara, his wife Mito Uzumaki, sealed it within herself.
After becoming Hokage, another village sent their best ninja to assassinate Hashirama, but Hashirama defeated him. He died of illness not long after the formation of the village, and his younger brother Tobirama succeeded him as Hokage.
Hashirama's presence in the world wouldn't end with his death. Orochimaru played around with Hashirama's and Tobirama's abilities, implanting Hashirama's cells inside of Yamato to give him Mokuton and discovered how to use Tobirama's Resurrection to the Impure World which he used to bring back both of the Senju brothers, which impressed Tobirama. The reunion however was short-lived when Orochimaru forced them to battle Hiruzen. At the end, Hiruzen used Minato's Reaper Death Seal to kill him and Tobirama, trapping them in the Shinigami.
In the Narutoverse people's chakra possess a single elemental affinity which allows the individual to learn that specific element easier than any other, and use more powerful jutsus with that element than any other. However, due to his unique biology, Hashirama possesses two elemental affinities: water and earth. By utilizing both of these elemental affinities at the same time, Hashirama could create and control trees. He can make the trees grow rapidly from seemingly anywhere, even his own body. Hashirama could create forests instantly and even create flowering trees which pollen would fill the air and induce sleep. He also possessed the ability to heal himself without using handseals and could bend Bijuus to his will. In addition to all of this, Hashirama had great skill in genjutsu and showed the use of "Bringer of Darkness," a genjutsu that would envelop his opponent in pure darkness which robbed them of their sight.