long-press an email and the trash can will appear at the upper right. Long-press more of them to delete more than one - then hit the trash can. (They go to trash, so hit the 3 bar menu [upper-left] and scroll down to Trash once in a while and empty that.)
For emails you don't ever want, you can go to
Gmail, check an email you never want to see again, click the 3 dots at the top, choose Filter, say okay to the first screen, then check Delete (and others like this - if there are any) and say okay.