Immediately I WAS in the spirit;

Paul had a similar experience in 2 Corinthians 12:1-4. When we are caught up to meet Christ, it will be instantaneous too (1 Corinthians 15:51-52).

and BEHOLD
 
• a throne was standing in heaven,

John still is amazed at where he is and what he is seeing. He uses the word, BEHOLD, again. The most prominent feature to catch John's eye is the throne. This throne lasts forever. It lasts through this current earth and heaven, the future new earth and new heaven, and forever. See Revelation 4:9, Revelation 20:11, Revelation 21:3, and Revelation 22:1.
• and ONE sitting on the throne.

The throne denotes the seat of government. It is the place where the King or ruler sits. In the Old Testament, the prophet Micaiah's vision was of this throne and God sitting on it -- 1Kings 22:19 and 2 Chronicles 18:18. In Psalms 47:8, God reigns over the nations from His holy throne. Isaiah had a vision of the throne (Isaiah 6:1-5) and so did Ezekiel (Ezekiel 1:26-28). The ONE Ezekiel saw was so awesome that he had to fall to his face. This is a common experience of all who meet God up close and personal.