In Act I, the girls are lying about what happened in the forest because they realize what will happen if it comes to be known that they were dancing around in the nude and having fun. Miller makes it clear in the expansive Act I stage directions that there is a strict code of conduct in how children must behave in Salem. We see this when the girls are alone, when Mary says that they will get "a whipping." In both of these, it is evident that the girls will be punished for their actions. At the same time, it is also evident that Abigail lies in order to maintain control over the girls and, eventually, others. They understand that if they accept responsibility and "come clean," they will be punished immensely. Yet, it becomes evident, especially at the end of the Act, that in lying and creating a distraction from their own actions, they will not only escape punishment, but will be looked at in the highest of forms for what they will do. It becomes a calculated move to lie and reap the benefits of it, regardless of the damage. At the same time, the girls are forced to lie because Abigail demands that all of the girls follow her lead and "stick to the story." The benefits of lying and the force of Abigail compels the girls to be deceptive and to distract others with the claims of witchcraft.
To understand why the girls lied about what happened in the forest, one should understand a bit more about the culture of Salem during that time in history. There were strict codes of conduct stemming from a religious standpoint. Attending church house services was mandatory, and every action of the people was to reflect one's devotion to God. That in itself is not a problem necessarily; however, any deviation from the prescribed code of conduct was met with severe punishment. Having read the story, I suspect your are already aware of that. What the girls did in the forest that night (dancing naked in the forest before the moon, and if I am not mistaken drinking blood) would be considered by the church leaders as pagan activities - tantamount to worshiping Satan - an indiscretion punishable by death.