Using a Weapon as an Implement
Some classes can use certain magic weapons as implements. If you're able to wield a magic weapon as an implement and use an implement power through it, you add the weapon's enhancement bonus to the power's attack rolls and damage rolls, but you do not use the weapon's proficiency bonus. If you score a critical hit with the magic weapon when using it as an implement, you use the weapon's critical hit effect.
-- "Character Options", PHB2 p. 204
That's the most concise summation of rules which are restated in several places. Suppose you're a level 1 draconic sorceror with a 16 Strength and an 18 Charisma, and you are using a +1 wyrmtooth dagger as an implement.
If you use the Dragonfrost power, your attack is +5 vs. Fortitude (+4 from Charisma, +1 for the weapon's enhancement bonus) and you deal 1d8 + 8 damage (+4 from Charisma, +3 from Strength for Draconic Power, +1 for the weapon's enhancement bonus).
If you score a critical hit with the Dragonfrost power, you deal maximum damage with it (16) and add additional damage and/or effects based on the Critical: entry in the weapon's stat block.
In the case of the wyrmtooth dagger, that's +1d6 damage per plus (so, +1d6) and the target loses all of its resistances, save ends.
Yes, you can use a level 2 +1 wyrmtooth dagger even when you're level 1. Level only restricts a few things about a magic item:
There's no restriction on using or acquiring items based on their level, except that you can't use the Enchant Magic Item ritual (page 304) to create an item above your level. If, for some reason, your 10th-level character finds a 20th-level magic sword, you can use it to full effect.
-- "Magic Items", PHB p.223, emphasis mine
However, if your DM is going by the treasure guidelines, you're only likely to receive magic items within 4 levels of your current level.