Obviously, depending on the starfish the food chain could vary. Say for example though, the British Common Starfish, a simple food chain would go phytoplankton -> Mussel -> Starfish.
If you want a diagram of a basic food chain/web, you can find one on this website here. (Just note that it's about a quarter of the way down the page and involves the starfish).
If you want a diagram of a basic food chain/web, you can find one on this website here. (Just note that it's about a quarter of the way down the page and involves the starfish).