In general, the Fourth Amendment dictates that searches and seizures conducted without a warrant are unreasonable and therefore illegal, meaning evidence secured would be excluded from any criminal proceedings.
As such, a law enforcement official must typically appear before a judge or magistrate, and demonstrate that probable cause for a search or seizure exists.
The judge or magistrate will then consider whether this is true based on the totality of the circumstances and, if so, issue the warrant. Of course, there are exceptions to this warrant requirement.
If you have been charged with a felony or misdemeanor, and believe that your rights have been violated in any way by law enforcement officials, consider speaking with an experienced legal professional.