The police knocking on your door can be concerning, especially if you have never dealt with them before. How you act in the next few minutes could make a big difference to your future.
Your first question is probably whether or not you need to open the door.
The police should tell you if they intend to enter anyway
The police have the right to enter your property in certain circumstances. However, you cannot just assume this is one of those occasions. You need to check:
If they have a valid warrant: If the police have a warrant that is correctly filled out and signed then opening the door for them is best. You can ask them to post it under the door or hold it up to a window so you can check first.
If they are in hot pursuit of a suspect: A breathless officer arrives at your door claiming a suspect has just entered. You can try telling them they are wrong, but if true it would give them the right to enter.
If they believe evidence is about to be lost or harm done: The law allows officers to enter without a warrant if necessary to prevent someone from being harmed or evidence being lost. Whether they were justified in their suspicions can be debated in court later if it proves they were wrong.
Why not just open the door as soon as an officer turns up?
Once you open the door an officer may use their foot to make it harder to close. They will have a better view of your property, allowing them to look for something to use against you. Opening the door also makes it more likely you talk to them, and it is easy to say something wrong that lands you in hot water. It also makes it easy for them to persuade or trick you into giving them permission to enter.
If you are unhappy about the way the police entered your house, seeking legal guidance could help you use it to defeat any charges that result.