Can the police get a warrant if someone told them I was doing something illegal?
Yes. These people are called confidential informants, tipsters and/or snitches. There are many rules surrounding the " tip" from another person. Again, rules vary from state to state and change over time but the general rule that assists most judges is how reliable or honest an informant may be.
The informant’s reliability or credibility is determined by many things; the same criteria you may use in your own life to decide if someone is believable or not. For example, has this person given information before that was relied upon? Has this person given other information about the present crime that has turned out to be true? Some of the most credible informants however, are "citizen informants," just ordinary bystanders who happened to witness a crime or actions that are highly suspicious of criminal activity, such as a neighbor noticing that stoned looking people come and go at all hours and in high volume to and from your home.