You don't state whether you live in UK, however in the US you are or soon will be of legal age in most States. That means that you can set to paper a simple will stating your requirements. This can be a brief, simple statement witnessed by two or more people (signing and dating the document) and then assure that it's kept by someone who will see to it's use.
Congratulations on your desire to donate your organs. Since you want to be cremated, the will should also state that a plain cardboard container should be used for the cremation, rather than a more expensive wooden or metal container. They are not required. Additonally, the container for the ashes that are then presented to the family or friends can be a simple cardboard or wooden box, rather than a more expensive urn.
The submitter GuavaHalf is entirely correct that services are for survivors, so you can instruct them to get together, have some good food and each can talk about their remembrances.
Hope this helps...