"seni" is accusative, used for the direct object.
"sana" is dative, and is used for the indirect object. Often, "to you" is a good translation, e.g. "sana kitap verdim" = I gave a book to you = I gave you a book.
However, some verbs take only a dative object, including benzemek "resemble", dokunmak "touch", and bakmak "look". (Perhaps it helps to think of them as "look similar to, lay hands on, look at which use prepositions rather than taking an object directly.)