To piggyback on what Patrick said:
You don't have to agree with every detail of Presbyterian doctrine, but you should be aware that they will be teaching you from the perspective of Westminster (hopefully, assuming it is a properly confessional congregation) and discipling you in that direction. If you have strong disagreements, you may find that this gets difficult for you over time. One of the membership vows that you will take is that you will seek the peace and purity of the church. The essence of this vow is that you will not actively be an agent of disruption and dissention in the congregation, but rather an agent of peace and unity. The thing is that if you have disagreements with Presby doctrine, you would be expected to not undermine the ministry of the local church by trying to convince others in the congregation that their church is in error - after all, PCA doctrine is historic, public and available to all, if the church is in line with the Westminster Confession then it is being faithful to it's calling. However, if the Pastor goes "off the deep end" and starts violating those doctrines which he has vowed to uphold, then there is another issue.
In other words - you are free to join the church given that both you and the Session have discussed the doctrine of the Westminster Standards and you are willing to come under their care and leadership. But as Patrick said, it might be best to make the Session aware of your doctrinal differences in your new member's class prior to taking the membership vows.