Custom template tag to activate a WordPress plugin

Hi,

I have been modifying a code to create my own plugin and I am unable to add custom template tags. I have been trying to use the add_filter function since yesterday but I do not know how I could call my function if I use a simple tag like other plugins do.

For example, I want my plugin to be called if [shopping_cart] appears in my post.

Can someone tell me how I can create such a tag that call my function??

Thank you for reading my post.

Regards,
G.C.Reddy