A melee attack can be made with a weapon or unarmed. The target has to be in a square threatened by your weapon. Threatened areas are all squares in reach of the weapon you are using. For melee attacks, your attack bonus is equal to your Weapon Skill bonus minus your scale.
Your target must be in your reach in order to attack it. Humans have a natural reach of Close, meaning they can use unarmed attacks against targets in their occupied area only. Most melee weapons have reach of Standard.
Close: In your occupied area
Standard: Areas adjacent to your occupied area
Long: Areas one square away from adjacent to you
Long X: X squares away
You can't generally use a weapon to attack an enemy closer than their reach without a special technique or ability.