go get your gameboy player located in celadon city. theres a building in between the Pokemon center and the pokemarket. go into the building and ride the elevator to the third floor then lyra will explain the rest!
You go to mount silver and fight ash with his very, very high level Pokemon. If you want to beat him you should have all your team members at minimum of of lv.70. Beyond that- there isn't much else....
There's a lot you can do. First of all, you can go and talk to Professor Oak. He'll open up the road to Mt. Silver for you. In order to do most of the other things, though, you have to defeat the...