First you must go to the Game Corner there in Celadon City. There in the back you should see a Team Rocket grunt standing in front of a poster, you have to battle him. After that he'll leave into their secret base underneath the Game Corner.
All you have to do is stand in front of the poster an push A to reveal the stairway down. Go through their base until you find Giovanni. Battle him an when you defeat him he'll drop the Silph Scope. Next you'll need to travel to Lavender Town an go inside the Pokemon Tower.
If you've gone inside here before without the Silph Scope you won't see any wild Pokemon just ghost's. If you haven't been inside yet you'll battle your rival. You'll need to go to the very top of the tower. When you reach the last stairs up a ghost will stop you, the Silph Scope will reveal it as a Marowak. You'll have to beat it, you won't be able to catch it.
After that go up the stairs an beat all the Team Rocket grunts. You'll see an old man at the end named Fuji. He'll thank you for rescuing him an takes you back to his house. He thanks you again for putting the spirit of Marowak to rest, an rescuing him from Team Rocket. To show his thanks he gives you the pokeflute.
Hope I helped!