What we hold to is captured in the historic standards of the Westminster Assembly produced in 1647. These consist of the Westminster Confession of Faith and the Westminster Shorter and Larger Catechisms. You can find links to them in the submenu to the left.
If you want a simpler statement of what Presbyterians believe, then Benjamin B. Warfield's A Brief and Untechnical Statement of the Reformed Faith is well worth reading. See the submenu to the left.