How can I find trash on my iPhone?

Trash as in Junk Mail?

It is in the mailbox concerned under Junk or Trash.

If you mean bogus System Other data which is old caches, go to Settings, general, iPhone storage and it will be at the bottom. It usually self-purges but it can go wrong with naughty apps.

You cannot get into it to look, only see the size, but if it is greater than 6 to 10 GB then Backup iphone, Restore to factory then restore the backup.

Allow a couple of hours: 

How to back up your iPhone, iPad, and iPod touch - Apple Support 

Restore your iPhone, iPad, or iPod to factory settings - Apple Support 

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

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