Install a security application
If you have not already done so, first thing you should do is install an application to protect your smartphone. Not only will help you to explore your device for malware but protect you from attacks from "phishing" (malicious web trying to deceive pages), and also tend to have other tools like filters against spam, functions burglar or even to establish parental controls, very useful, for example if it is a tablet using your children.
Typically this should be enough to clean your smartphone. Note that antivirus may give some kind of "false positive" indicating as malicious any app you have used for months without problem. In that case you can investigate other opinions of the app in question or simply ignore the warning if you think it is a safe program.