Determining Attribute Scores
There are different ways to determine your attribute scores, and the Heroic Die Method is most common. For this you roll 4 six-sided dice (4d6), remove the die with the lowest roll, then add the three remaining dice together to get your score. Continue this five more times until you have six scores (one for each attribute). The highest score you can achieve is an 18, and a 3 the lowest score.
Placing Attribute Scores
Once you have all six scores determined, you assign each one to an attribute. Typically, you will want to assign the highest scores to those most important to your Class (listed in the Class traits box as "Primary Attributes"). For example, as a Wizard, you will likely place the highest score in Intelligence. For a Fighter, Strength is typically most important.
Also, keep in mind your character concept when placing your Attribute scores. If you want your character to be a leader or social butterfly, a high Charisma score is important. If your character has a bit of a clumsy streak, a low Dexterity will show that weakness. Remember that the highest scores will represent your character's strengths and the lowest scores his weaknesses.
Your rolled attributes can be further modified by your character's Race.