Answer the Door
Obviously, if your doorbell rings, you will go see who is there. If you see officers or detectives with badges, ask why they are there.
Avoid Making a Statement
Officers will have questions for you inevitably if they are at your door. Always ask why they are there and what they need from you. If they want to know if you saw anything, you can answer. If they insinuate that you were involved, immediately stop answering and call a lawyer.
Do Not Let Them In
Your home is your castle. If officers want to enter your castle, they must get a search warrant. There are very few sacred rights in this world, but one of them is to have privacy in your house.