When you delete a message, it move to your Trash. After 30 days,it will be permanently deleted and can't be recovered. Learn to delete or undeleted messages here !
STEP 1 : Sign into Gmail account. ( Learn more Gmail Sign In guide )
STEP 2 : Choose the messages you want to delete.
STEP 3 : Click `Delete `
Becareful, when you empty your trash, you can't restore the deleted message.
STEP 1 : Go to `Trash` : Click `More` → `Trash` on the left.
STEP 2 : Select the message to delete forever or delete all message.
You can restore the deleted message in the last 30 days by `Move them to another folders / labels likes Inbox, Private, Promotion,...