Firstly: yes, "I" = "ja", but in Polish you do not need to use the personal pronouns much. In fact, you usually omit them, especially for 1st and 2nd grammatical person. You can write "Ja lubię mleko i chleb", but it sounds as if you were emphasizing I.
Secondly: "lubią" is only one of the forms of "to like". In Polish every grammatical person has its own form of the verb, and therefore you may not use the pronoun, as the verb makes it obvious which grammatical person is the subject. "lubią" is 3rd person plural: they like. "lubię" is 1st person singular: I like.