。
Have you decided whether to come with me?
Literal
I [with-と] together come decision [topic-は] attached-[question-ましたか].
決心がつく is a set expression — '[a] decision comes to stand/attach' = 'to resolve, to make up one's mind.' 決心 is the noun 'resolution,' つく is the intransitive 'to attach, to come about.' Contrast with 決心をする ('to make a resolution') where the person is the agent. ついた form marks the decision's arrival.