I cannot understand how to properly fry seafood
Given your picture, I think the correct term here is saute; that is, to quickly fry in a little bit of hot fat. Pan frying uses more fat, and a lower temperature, to create a deeper crust. Probably an overkill for mussels.
For a saute, you want relatively dry (pat dry with towel if necessary - frozen seafood can release a lot of water) ingredients, relatively high heat, and a solid pan that is not over-crowded.  Heat the pan, add the fat, then the ingredients.  Then generally keep the ingredients moving during the cooking. Here is some further direction that details the process.
See also this for a discussion of the origin and use of term/technique.