u get the HM surf and teach it to one of ur Pokemon. but make sure to beat the hearthome gym, that will allow u to use surf outside of battle. then go to jubilife city and keep heading west. go through to passage thingy and come out to a pier like area. go the northern way because if u keep going west, ull just end up and a strip of land backed up by trees. theres gonna be 2 fishermen there that u can fight. the other way leads to the main road, theres gonna be a sailor there and u have to fight him. go down and ull see a guitarist that is standing there. u can either fight him or go through the grass to get to the berries on the other side. go west and u will see another passage thingy that will get u to canalave city! but make sure to heal ur Pokemon first! as soon as u get on that bridge, ur rival will show up and challenge u! good luck and i hope this helped!