。
I like soccer more than baseball.
Literal
I [topic-は] baseball [than-より] soccer [subject-が] like-[attributive-な]-[explanation-んだ].
The standard comparison: XよりY(の方)が好き ('like Y more than X'). Here the 方が is omitted, a common colloquial shortening. 好きなんだ uses な (attributive copula for な-adjectives and nouns) before んだ. サッカー is an English loanword; 野球 ('baseball') is a native-style compound (野 'field' + 球 'ball').