For most people, getting to Jyväskylä means first getting to Helsinki, the capital of Finland. Flights to Helsinki are available from all over the world. It is also possible to take the ferry from Stockholm, Tallinn, Rostock or Travemünde.
When booking the flights, choose Helsinki-Vantaa airport. There might also be a few options to choose a flight to Jyväskylä Tikkakoski airport but you may need to change the plane in Helsinki.