The police can enter your home if they have permission from you or someone else with authority over your home, if they have a warrant, or under certain urgent circumstances such as investigating a 911 call, if they believe someone is in immediate danger, or if there is evidence in your home relating to a serious offence and there is a risk that evidence will be destroyed during the time it would take to get a warrant.
If police enter your home, they should first knock and tell you why they are there. They do not have to do this if it may lead to someone being harmed or evidence being destroyed. If they have a search warrant, they should show it to you. You should check to make sure the information on the warrant is correct. You can tell the police you do not want them in your home, but you should not try and stop them from entering. If you believe police have entered your home unlawfully, contact a lawyer immediately.