For present tense, it's really easy: There is no present tense for perfective aspects of Russian verbs. мыть is imperfective, помыть is perfective, so you can only use мыть here. You literally cannot use помыть to translate this sentence.
As explained in
Imperfective - Incomplete, ongoing, habitual, reversed or repeated actions
Perfective - Actions completed successfully.
When you're talking about present tense, activities have to be on-going. It doesn't make sense to talk about something going on at the present moment which has been completed and thus is not going on at the present moment.
По is a prefix. This time, помыть (or вымыть) is a perfective counterpart of мыть. It means a single action finished at some point in time. You use these to express finished actions in the past or the future and also to tell stories in a sequence (i.e., when one action follows another).
Мыть, on the other hand, expresses a range of more blurry meanings like ongoing, repeated or habitual action, the name of the action or action mentioned without focusing on when exactly it happened.
English does not have this perfective/imperfective distinction in its aspectual system, so understanding the basics of it will take you some time.