Trinitarian's conclude that the Trinity is proven by the use of "Elohim" in its plural use in the Old Testament, such as in Genesis 1:1 
"In the beginning God [Elohim] created the heavens and the earth."

"Strong's Exhaustive Concordance:

angels, exceeding, God, very great, mighty
 

Plural of 'elowahh; gods in the ordinary sense; but specifically used (in the plural thus, especially with the article) of the supreme God; occasionally applied by way of deference to magistrates; and sometimes as a superlative -- angels, X exceeding, God (gods)(-dess, -ly), X (very) great, judges, X mighty"

Debunked by the notable Trinitarian teacher, Norman Geisler.  
 

Use of Plurality - Norman Geisler and Thomas Howe - “Others have claimed that the plural pronoun refers to the Trinity. It is clear from the NT (e.g., John 1:1) that the Son was involved in the creation of the heavens and the earth. Also, Genesis 1:2 indicates that the Holy Spirit was equally involved in the creation process. However, students of Hebrew grammar point out that the plural pronoun “us” is simply required by the plural Hebrew noun elohim which is translated “God” (“Then God [elohim, plural] said, `Let Us [plural] make man in Our[plural] image’ ”). Consequently, they claim that this statement should not be used to prove the doctrine of the Trinity. Indeed, the Quran, which denies more than one person in God, uses “us” of God, demonstrating that the Near Eastern usage of “us” does not necessarily mean more than one person.
 

Still others have asserted that the plural is used as a figure of speech called a majestic plural. In this use, God is speaking to Himself in such a manner as to indicate that all of His majestic power and wisdom were involved in the creation of man. As has been noted, the plural pronoun “Us” corresponds to the plural Hebrew word elohim which is translated God. The fact that the name “God” is plural in Hebrew does not indicate that there is more than one God, or that this is actually a reference to God being a group of extraterrestrial astronauts. There are a great number of passages in the NT that refer to God with the singular Greek noun theos, which is also translated “God” (Mark 13:19; John 1:1; Eph. 3:9; etc.). The plural nature of the Hebrew word is designed to give a fuller, more majestic sense to God’s name."    Full Article HERE