The first question may be "Why do you think your phone has malware?" Bear in mind that this diagram, while not universally correct, is a good default assumption.
Assuming your phone is not jailbroken, an antivirus app would not have the necessary permissions to perform most of the tasks you would expect of such software (such as scanning installed apps, detecting malicious software in real-time, or repairing modified system files). Similarly, unless malware contained something similar to a jailbreak hack itself, it is unlikely that it could significantly interfere with the phone's operation.
The most likely impacts of malware (for non-jailbroken phones) are things like racking up high phone bills through premium calls or SMS (a common problem on Android; not sure about iOS), tracking your location, or possibly injecting browser ads or spying on browser activity. These are all things that a malicious app could potentially do (although the risk of such apps is relatively low, it is not zero). Simply uninstalling the relevant application should fix the problem.
If your phone is jailbroken, both the likelihood of malware and its potential impacts increase significantly. Detecting and removing it is likely to be more difficult as well. The best fix in that case may well be to hard reset (completely wipe) the phone.
Resetting the phone is also probably the best approach to fixing an unexplained issue that isn't caused by malware. Make sure your data is backed up first!