The correct order to secure your wedding services is
1. Ceremony Location
2. Reception Location
3. Photographer
4. Music
5. Wedding gown/ Bridesmaid dresses
6. Wedding Cake
7. Florist
8. Limo
9. Groom/Groom's attire
10. Invitations
11. All remaining incidentals such as candy bar, band, favors, etc and dont forget the Honeymoon!
Here is a great Wedding Day Check list
All of this great information and more is on weddingplanning.org