Important points to remember with regard to Sola Scriptura (and its defense) in no particular order:
1.) Sola Scriptura is the statement that Holy Scripture is the only infallible rule of faith given to the Christian church, not that Scripture is the only rule of faith. Creeds and confessions are rules of faith, but the authority they have is derivative. What this means is that the reason why they have any authority at all is because they are an accurate summary of Biblical teaching, but they are not infallible, meaning they can (and do) err.
2.) The idea of the Scriptures being the only infallible rule of faith flows from the fact that the Scriptures are the only source of God-breathed, or divinely inspired, revelation the church currently possesses. The Scriptures are the very voice of God (2 Tim. 3:16, Matt. 22:31-32) and thus have authority over all human teaching. Since nothing else possesses this divine quality today, they are the only infallible rule of faith.
3.) Closely related to the last point is that Sola Scriptura is a statement of authority with specific respect to the current state of the New Covenant church post AD 70. Obviously during the times of living prophets and apostles, there were other sources of divine revelation besides the Scriptures.
4.) Sola Scriptura is a statement of epistemological authority, not a statement about hermeneutic. And so, when critics point to variations amongst Protestant doctrine Sola Scriptura is not to be blamed for this. Citing the Scriptures as your ultimate authority does not automatically determine exegetical conclusions.
5.) The primary sufficiency of the Scriptures is seen in theological and spiritual applications (2 Tim. 3:16-17) for the elders and teachers within the church. This does not mean the Bible's authority is limited only to the spiritual realm, it simply means that when Scriptural sufficiency is confessed, faith and godly living are primarily in view.
6.) The Scriptures are perspicuous, meaning they are clear and easy to understand. Not that every single part of the Scripture is as easy to understand as other parts, but that a simple person with an adequate reading comprehension can take and read, and apprehend what he needs to know in order to have peace with God and live in a manner pleasing to Him.
7.) In the same way that Scripture's authority comes from its Divine inspiration, the canon of Scripture is understood simply to be those books that God has inspired as opposed to other books. The canon is not created by the Church, but rather it is received. By the internal witness of the Holy Spirit, Christ's sheep hear the Shepherd's voice in the sacred text.