First things first:
Before importing your Vanguard from ME2, go into ME2 and load your Vanguard save and reset your talent points. This will allow you to assign all your talent points from scratch in ME3.
Now on to the most commonly-used powers:
Charge: Radius > Weapon Damage > Barrier
Nova: Damage > Half Blast > Pierce
Incendiary Ammo: Squad Bonus > Headshots/Ammo Capacity > Explosive Burst
Commonly-used bonus powers:
Reave (requires Kaidan to unlock): Radius > Damage Reduction > Damage and Duration
Fortification: Durability > Power Damage > Durability
Most commonly-used weapons:
Claymore shotgun
Raider shotgun (DLC)
Reegar shotgun (DLC)
Piranha shotgun (DLC)
Talon pistol
Hurricane SMG (Collectors Edition/modding)
Punisher SMG (DLC)
And I would probably throw the Harrier assault rifle in that list as well.
Levelling up your Vanguard:
1) Level up Charge first
2) Nova to half blast
3) At least one point into Incendiary Ammo, rank 6 for Explosive Burst is good.
4) Probably Assault Mastery next.
5) Fitness last unless you want more health and shields sooner.
6) Bonus power last