Things useful when writing a song:

• Good Health
• Good Musical talents
• Be in your Apartment or a Hotel while composing, except, of course, if you are trying to get Inspirational Seeds by partying, then you should be in a Club. This is not recommended if you'd like to write several songs, as your health drops down very fast and it's usually not worth it.
• Basic Composing and Basic Lyrics both 5* or higher
• There are several other composing skills that help a bit too, like for an example: Composing Crowdpleasers and Composing Floorfillers. You can select one of those skills in your songs settings to use it, they only improve the quality of the melody and they are private skills.

Additional guide: Tempos used in songwriting