In short
Trinity
There is one God: Father, Son and Holy Spirit, a unity of three co-eternal Persons. God is immortal, all-powerful, all-knowing, above all and always present.
Read it in the Bible
Deuteronomy 6:4; Matthew 28:19; 2 Corinthians 13:14; Ephesians 4:4-6
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.