Try going to your school guidance counselor and let them know you are interested in shadowing an SLP, your school may very well have a relationship with someone in this field.
You can also check online for clinics and hospitals in your area that offers services in this filed. Email them and let them know that you are interested in this field ask if they have a program that will allow you to shadow an SLP. Often times even if they don'y have a program they will be able to tell you where to get started. Good Luck and I am positive you will make this happen!