Revelation 19:6 - "And I heard it as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying Alleluia: for the Lord God omnipotent reigneth."

There we go, in the context of this passage we can read that God is omnipotent.

God has the power to create anything he wants to.  In fact, he did that in Genesis and He's been doing it for all eternity.  In fact, God even created time.  There was no such thing as time before the creation of the world. 

Genesis 1:1 - "In the beginning, God created the heaven and the earth."

The verse doesn't say directly that there was no time, but it does say "In the beginning..."  The beginning of what?  The beginning of time as an entity.  God transcends time and space, which is why He can be anywhere and everywhere at any given time (omnipresent) and also exists in the future as well as the present and past (omnitemporal).

God is not affected by time.  He created it, and is completely separate from it, otherwise God would have an end and a beginning, when scripture states that he is the beginning and the end.  In a sense, God is independent of time, but time is not independent of God.

Revelation 1:8 - "I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty."

God is also eternal, as is stated below:

Psalm 90:2 - "Before the mountains were brought forth, or ever thou hadst formed the earth and the world, even from everlasting to everlasting, thou art God."

So how does this prove that God is all-powerful (omnipotent)?  Well, He can do anything He wants to, and isn't that really the definition of all-powerful?

Omnipotent
1. Having unlimited or universal power, authority, or force; all-powerful.

(The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.)