In short
Holy Scriptures
The Bible, Old and New Testament, is God's written Word, given by inspiration to holy men who spoke and wrote as the Holy Spirit moved them. It is the standard of character, the test of experience and the trusted revealer of what we should believe.
Read it in the Bible
2 Peter 1:20, 21; 2 Timothy 3:16, 17; Psalm 119:105; Isaiah 8:20; John 17:17; Hebrews 4:12
This is a short summary. The church's full statements are at adventist.org, and our pastors are glad to study any belief with you.