if your national organization is a member of an international organization:

first check if they have already signed a partnership agreement with IYCP at an international level, in which case you may contact the international headquarters to benefit from the partnership already established, or, if they have not signed such an agreement, you may suggest that they become associated with the Movement by doing so

in any case:

look at the contents of the National Partnership Agreement
 
and

contact your National Focal Point in order to sign it