Revelation Chapter 4, Verse 2 (NIV)

^4:2At once I was in the Spirit, and there before me was a throne in heaven with someone sitting on it.

At once I was in the Spirit,

Humans are bound physically by time and space. So this "spirit dimension" is unique and not common for man. It is, however, the dimension(s) in which God moves. The transformation into this dimension happens immediately.

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 there

before me was a throne 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.
with someone sitting on it.
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 (1 Kings 22:19 and 2 Chronicles 18:18). In Psalm 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 close and personal.