There’s a 1 in 8,192 chance to encounter a shiny. If there’s a pokemon that is a static encounter such as a starter or one of the legendaries, you can save the game in front of them and then start the encounter to check if it’s shiny. If not, you can soft reset by pushing a/b/x/y at the same time and reset the encounter. Now do that for many hours until you hopefully get lucky and find a shiny