This may seem like putting the chicken before the egg, but once you decide the genre and style that your band is going to develop and play, you are creating the direction of the band.
This should make it pretty obvious that the direction of the band will be one of your very first priorities.
You probably already have in mind the genre (rock, pop, country, R&B etc.
) and style (metal,punk,bluegrass etc.
) of music that you want your band to be, so at this point you will need to begin creating a style appropriate song list.
If you're a cover band (playing other bands music), you need to search for the bands that you desire to sound like and build your song list from there.
Always keep this in mind though, it's not to in your best interest to pick songs that are out of the bands musical scope and/or singing ability.
It's also probably not very wise to choose songs or bands that have a lead singer that has a five octave range and/or the guitar player shreds like Eddie, if you can't pull it off musically.
Those songs are great goals for later, but try to stay and create a sound that is with in your musical scope.
It's possible your desire is to play all original material (songs you compose).
Go a head and start making that song list as well.
At this point of your musical career, you may have more desire than songs to fill an all original song list.
There is nothing wrong with that.
But to get the process started, create dummy or a working titles list.
Again, these may be songs that are written by other bands or just made up dummy titles.
Just get the process started.
There might even be songs that you have written or are  contemplating writing.
Create the list.
Get the process started.
Probably the biggest point  I am trying to stress here is, stop  dreaming and get the process rolling.
Did I say that already?  This is the best time to decide what band or bands you want to style and mold your band after.
I know that you are probably saying that your band is going to be "different from anything ever before", and that's cool, but you should find some songs that you know your band mates can begin to jam, play and gel together with.
This is easily the best way to help you organically grow into your "sound".
  Even though you may not want to be a cover band, you would be well advised to pick and play five or ten songs from other bands you admire.
This will help you in the early developmental stages of your bands career.
One, it will get the band jamming and getting tight with each other musically.
And two, your song list will begin to grow and change as your band grows and changes.
So remember this.
You are creating the foundations with which your musical career is built upon.
The song list is a great place to start.
Have fun!