I was baptized when I was 13. I did it basically "because it was about that time". I grew up in a church that was very, um, antiseptic. Very rigid and unemotional. When I was baptized, I knew what I was doing, but only with my head and yes, I was afraid (I'm not sure of what).
Maybe you should pray for God to enlighten you, and help you to fully comprehend (with your heart) what you are doing - showing openly that you are giving your life to someone who gave His life for you. It's hard for us to comprehend how great God is and how great Jesus' sacrifice and love is. I can't comprehend it.
But may God help you to be able to put all your fears aside (as natural as they are) and understand His mercy and goodness.
I love these lyrics from Philips, Craig and Dean. They help me understand God's love a little better:
Almighty God, the Great I Am
Immovable Rock, Omnipotent, Powerful, Awesome Lord
Victorious Warrior Commanding King of kings, Mighty Conqueror
And the only time, the only time I ever saw Him run
(chorus)
Is when He ran to me, He took me in his arms
Held my head to his chest, and said my son's come home again
Lifted my face, wiped the tears from my eyes
With forgiveness in his voice he said son, do you know I still love you
It caught me by surprise
When God ran
The day I left home, I knew I had broken his heart
And I wondered then if things could ever be the same
Then one night, I remembered his love for me
And down that dusty road ahead I could see
It was the only time, the only time I ever saw Him run
(chorus)
It caught me by surprise
and it brought me to my knees
When God ran
I saw Him run to me
I was so afraid all alone and so far away
But now I know, He's been waiting for this day,
I saw Him run to me
(Chorus)