ה' שפתי תפתח ופי יגיד תהִלתך
Before we begin the 'amida, we recite a very special verse from the book of Psalms, (51:15) "God, open my lips, so my mouth would proclaim Your praise". To understand why the Rabbis chose this verse to introduce the 'amida, it is critical to see this verse within its original context. Psalm 51 is a Psalm of Teshuba (=repentance, contrition). David regretted the sin he committed with Bat-Sheba. When composing this Psalm, David was in a state of deep anguish, consumed by guilt and shame. As he was imploring to God with a broken heart, at one point he felt speechless and powerless. The reader can see that suddenly, the flowing of this Psalm stops. As if David, aware of God's Presence, became short of words to beseech God's mercy. He paused. Then, he said, God, as I wish to continue praying to You, I realize that I need Your help to address You, please, give me the strength and courage to address You, "God, Open my lips, so my mouth would proclaim Your praise". With this extraordinary request David haMelekh conveyed the depth of his contrition and his unique humbleness. But above all, he taught us that even the most gifted and eloquent biblical poet, when realizing that he is talking directly to God, he requires God's assistance.
The 'amida is not a regular prayer. While in all other prayers we talk about God, in the Amida we are talking to God. Talking about God, praising Him, recounting His blessings as we do, for example, when we read the Psalms in the first sections of our prayers, is somehow within our abilities. Talking directly to God, however, when seriously taken, might/should be an overwhelming experience. When we recite the 'amida we are not repeating the words of David haMelekh. When saying the 'amida, in a sense, we are David haMelekh. Upon realizing what we are about to do, we are overcome by reverential-fear and a sense of inadequacy and powerlessness to experience His Presence; let alone to address Him with words.
This verse was chosen by our Rabbis for the introduction of the 'amida. To inspire us to feeling the humbling and overwhelming experience of addressing directly God Almighty.