Basic magic weapon and implement rules:
- The enhancement bonus applies to attack and damage rolls
- If the item is L1-5 you get a +1 ehnancement bonus, if it's L6-10 you get +2 and so on (every 5 levels).
- If you are proficient with a weapon as an implement you get the bonus for weapon and implement attacks. If the implement can be used as a weapon and you a proficient with it as such you get the bonus for weapon attacks.
- This isn't in your PHB because it was introduced later but there are three item rarity classes, Common (basic magic items), Uncommon (slightly more advanced) and Rare (very advanced). Common items are available for sale, Rare and Uncommon items are only available through adventuring. (See the Rules Compendium treasure tables for how to determine what rarity of item to award in a treasure parcel).
- Common items sell at 20% of value, uncommon at 50% and rare at 100%.
- Daily Item power restrictions have been errata'd in favor of item rarity
Now. As to how the bonus is applied, if you are Level 4 (which would the minimum level you could be and have an int of 21) then you'd likely have a +1 magic staff.
This means your attack bonus on implement powers that use intelligence would be as follows:
Int (+5) + 1/2 Level (+2) + enhancement bonus (+1) + other modifiers
And your damage if the power includes Int in the damage equation would be as follows:
Die + Int (+5) + enhancement bonus (+1) + other modifiers
This is all explained on page 274 of the PHB, there is a nice example there
Example: Melech, a 7th-level tiefling wizard,
attempts to hit three enemies with fireball, an Intelligence
vs. Reflex attack. His attack roll against each
target gets a +10 bonus, which includes +3 for one-half
his level, his +5 Intelligence modifier, the +1 feat
bonus from Hellfire Blood, and the +1 enhancement
bonus from his +1 wand of witchfire. He could add a
+2 bonus from his Wand of Accuracy class feature
against one of his targets and a +1 racial bonus against
any bloodied targets from his Bloodhunt racial trait.
there are several other examples in this section that illustrate similar concepts.