Asked at a workshop for advice on how to build a repertoire (of Irish traditional songs), I once suggested:

- resolve to sing ONLY songs that you really enjoy singing
- think of about 6 - 7 major subcategories of the genre (humorous, political, place-songs etc)
- identify and learn one SHORT and one LONG song in each category
- don't rush - these things take time!
- add new songs as you learn them but always think of this basic classification.

Armed with a repertoire like that, you can sit in any session and expect an opportunity to slot in an appropriate song, of appropriate length at an appropriate time. That way, you contribute to the organic nature of a good session. Few things wreck a session completly so much as a singer determined to sing "their" song, regardless of context.

Regards