When the officer is telling you that anything you say can and will be used against you, they mean it. Rules of evidence permit statements made by a criminal defendant to be admitted into evidence, whether the defendant testifies in court or not. Even if the statements do not seem to be harmful to your case, they very likely still may be. When you are being interrogated by the police, you have no idea what other evidence they may have against you. Unless you are an attorney or have done a substantial amount of research into the elements of each individual law, you probably also do not know what the State must prove in order to convict you of a crime.
As Darryl A. Goldberg stated in his 2008 article entitled, Responding to the Middle-of-the-Night Call from an Arrested Acquaintance, "A confession is the most powerful piece of evidence for the prosecution. What you see on TV certainly rings true in almost every case: whatever you say can and will be used against you." Confessions are often the main piece of evidence used to convict a criminal defendant. Keep quiet, do not confess to a crime or make any statements that could be used to prove that you committed a crime, and the State's job of proving you guilty is much more difficult. Moreover, research has shown that even innocent people sometimes make incriminating statements or outright false confessions. See this, this, and this. Don't take the risk that your statement may be interpreted as incriminating or that you may succumb to police pressure.
So how do you make sure that questioning ceases and that you will not continue to be questioned? Simply say: "I do not wish to speak with you. I would like to speak with an attorney right now." This ensures that you have invoked your right to remain silent, and also that the questioning must cease because you have requested an attorney. In addition, you should then speak with a criminal defense attorney as soon as possible, so that the attorney may represent your interests in dealing with police and with the State and ensure that you are not questioned further.