Determine whether you are a witness or a target/potential target of the investigation–neither the prosecutor nor you can give your attorney a definitive answer, since both know things the other doesn’t. You need an attorney because the prosecutor and police will tell him things they won’t tell you. Determine whether any privilege attaches to the testimony or documents sought. There are constitutional and legal protections for certain types of conversations and documents. You need counsel to determine whether any of these apply to your testimony. If you are a potential target, determine whether an offer of immunity will be made, or whether a specific bargain is available for cooperation.
Prepare with your attorney–be sure you know how to unequivocally invoke your rights, and how to consult with counsel when you need to. Your attorney cannot enter the grand jury room with you, but you have the right to ask to consult with him, and leave the room for that purpose.