Whre can I find sign language lessons for a 13 year old?

My cousin just turned 2 years old and she had an infection that made her deaf. I love her from the bottom of my heart and i need to learn sign language from her. SInce tomorrow is the last day of school for me i have all summer long to do this.I amm thirteen years old so where can i find sign language lessons for teens? I live in frederick MD so if anyone knows somewhere near ther that would be a great help! thanks!