This has been brought up a lot and no, you spawn with a "custom class." The class being
Main Weapon-BR, DMR, Carbine, Assault Rifle, or Promethean AR
Sidearm-Magnum
Grenade-Frag, Plasma, or Promethean Grenade
Perk
That is the formula that has been released but is subject to change, they have said that it will not change much more than that though. Considering how snipers are extremely powerful in Halo, it would just be silly to let us spawn with them.