Some djs are prepared and send out an organized planner for bride and groom to fill out. I send that out and have couple complete it on Google docs so it can be edited at any time. I also include a long list of most popular songs to help out brides and grooms pick their music. 

If you are having a dj provide the ceremony music, I always recommend to have about 30 to 45 minutes of pre-ceremony music. I typically start music 30 minutes before the ceremony but sometimes the bride and groom need extra time or maybe some guests have not arrived yet. 

The Pre-ceremony music I mostly play instrumentals but it can be anything the bride and groom can cook up.

The Processional music can be 2 to 3 songs for wedding party, groom, and bride. 

The Recessional song would be the song everyone exits to.

In your song selections make sure you add the must play songs that you want at your wedding. Make sure to include songs that your guests and family would like also. Maybe you can include the old classics your parents would enjoy for the dinner portion of the reception. Keep in mind during the cocktail hour your family and wedding party will be taking photos so make sure your must play songs will be played after the cocktail time. 

On the same note, please let the dj know of what songs not to play. Maybe you do not want the Chicken Dance played so make sure the Dj does not play it. I never play it unless the hosts ask me to. There maybe a few songs on that list or maybe include a genre of what not to play.