wayneman is correct. "Doctrine" means "teaching", "sound" means "healthy". So, we are to adhere to "healthy teaching", or "sound doctrine".
This includes both orthodoxy (right belief) and orthopraxy (right practice). One cannot be had without the other, and one cannot be substituted for the other.
Ultimately, sound doctrine is that teaching which accords with what we have been taught in the Scriptures by the Lord, Apostles, and Prophets.
There have been attempts in church history to write down a body of sound doctrine; these documents are usually called "confessions of faith".
Several Reformation era confessions that you may be interested in looking into are the Westminster Confession of Faith (Presbyterian), London Baptist Confession (1644 - Baptist), 2nd London Baptist Confession (1689 - Baptist), Belgic Confession (Continental Reformed).
Ancient creeds that were attempts to define a body of sound doctrine, or Christian orthodoxy, are the "Apostle's" Creed, Nicene Creed, and Athanasian Creed. These may also prove helpful to you.
In current times, there is a tendency amongst the revivalistic brethren to want to say "no creed but Christ", or "I just believe the Bible". However, sadly, because of the abundance of false teachings around us, it is necessary to define what we believe the Bible teaches by using extra-Biblical language.
Sound doctrine is very important to the practical life of the believer. There can be right believing without right doing. However, there can never be right doing without right believing.
You may find it helpful to read through a systematic presentation of Christian doctrine. Louis Berkhof's "Summary of Christian Doctrine" could be a good place to start. It is fairly brief and condensed.
Summary of Christian Doctrine