If something suits you, it is something whose style is appropriate for you, whereas if it fits, generally we are speaking about the size. However, sometimes they can be used interchangeably. You can say, "That tie really suits you." It means that the tie matches your personal style. You can't say "that tie really fits you." It would imply that some ties aren't the right size. You can say: "That shirt suits you well" or "That suit fits you well." But the meaning is different. The first is talking about the style matching your own personality. The second means the size is appropriate, it isn't too small or too large. You can say, "He was a good fit for the job" or "the job really suits him" and the meaning is the same: the job matches his qualifications. I hope that helps.