Where is the "Sea of Tiberias"?
"Sea of Tiberias" is another name for the Sea of Galilee, which is also called the Lake of Gennesaret. Tiberias is the resort city that Herod built on the western shore of this body of water and named after Caesar Tiberias.
Which and how many of Jesus' eleven disciples were there?
Peter, Thomas, Nathanael, James and John ("sons of Zebedee" - John 21:2) and "two others," so seven of them.
Why were they there in the first place?
They were heeding the angel at the tomb of resurrection: "But the angel answered and said to the women, 'Do not be afraid, for I know that you seek Jesus who was crucified. He is not here; for He is risen, as He said. Come, see the place where the Lord lay. And go quickly and tell His disciples that He is risen from the dead, and indeed He is going before you into Galilee; there you will see Him. Behold, I have told you.' So they went out quickly from the tomb with fear and great joy, and ran to bring His disciples word." (Matthew 28:5-8)
Why did they go fishing?
Since the angel had told them that Jesus "is going before you into Galilee," they probably arrived in Galilee expecting to see Jesus waiting for them. When they found Jesus neither upon their arrival nor after waiting, Peter decided to go fishing and was joined by the others, including two or three ("two others of His disciples" (John 21:2) could include Andrew, the brother of Peter) other fishermen by trade: "And Jesus, walking by the Sea of Galilee, saw two brothers, Simon called Peter, and Andrew his brother, casting a net into the sea; for they were fishermen... Going on from there, He saw two other brothers, James the son of Zebedee, and John his brother, in the boat with Zebedee their father, mending their nets..." (Matthew 4:18,21)