I just addressed this issue with one of my groups. Hopefully what I told them should help you out as well.
Attacks and Damage
Whenever you attack an enemy, there are two parts to it.
Attack - Determines whether or not you hit the enemy. Attack will ALWAYS be determined by rolling a d20 and adding any attack modifiers. Attack will always target a certain defense which you should read whenever you make the attack. If you roll a 13 and have a modifier of +4 and the skill is targeting reflex, you would say something along the lines of "17 versus reflex". There is another important issue with attacking. If you roll a natural 1 (1 on the d20) you just experienced a critical miss. No matter how high your modifier (even if it's above the enemy's defense) you cannot hit them. Conversely, if you roll a natural 20 (20 on the d20), you just scored a critical hit. You are guaranteed to hit the opponent and deal the maximum amount of damage possible for that attack.
Damage - After you get a higher number than the enemy's defense with an attack you roll for damage. Damage is stated on the card (usually through dice rolls + a modifier). Some attacks may include 1[W] (or 2/3/4 [W]) as part of their damage. This means your weapon damage times the number before it. So if an attack deals 1d8 + DEX and you have a dexterity modifier of +3 and you roll a 6 you will deal 6+3=9 damage.
These are the cards you find on your character sheet. They give you all the information you need to use a power.
Action Type - States what type of action the power is. Most likely this will be a standard action for an attack, but always read it to make sure.
Attack Modifier - This is what you add to your dice roll to determine if your attack hits. It is also displayed at the bottom of the card for each weapon you have equipped, I would highly recommend using the number next to your weapon because it will always account for any bonuses your weapon may have.
Targeted Defense/ Versus Defense - This is the defense your attack will target. Always let the DM know what this is when you make your attack.
Damage - This is the damage your attack will deal. Once again, it is also displayed next to the weapons you have equipped and it is recommended you use this number as it will account for any bonuses your weapon may have UNLESS you weapon deals more damage in certain situations such as when an enemy has no allies adjacent to it, these bonuses must be remembered and added after the fact.
Range - This is how far your target may be from you. This will most likely say "Melee 1" (one square away from you) or "Ranged X" X being the number of squares away that you may fire. It may also include the words "Close burst X", "Area Burst X" or "(Close) Blast X". All of these denote areas of effect (powers effecting more than one square) and you can read what each of these terms mean on pages 271-2 of the Players Handbook 1.
Target - What can be targeted by the power. Creature means absolutely any creature, friend or foe. Take this into account when using area of effect spells as they may harm your allies as well.
Power Type - This will most likely say "At-Will", "Encounter", or "Daily". This lets you know how often you are allowed to use the power. At-Will may be cast whenever, Encounters recharge after a short rest, and Daily Powers are recharged after an extended rest. This area may also say "Utility Power" in which case there should be a box elsewhere on the card that has all three types written down and a checkmark next to the type that the ability is.
Effects - Many attacks have more effects than just damage. These usually appear in the area labeled Hit, although they may also have their own separate section on the card. When you hit with an attack these effects also take place.