If you opened an attachment...
An unexpected attachment - likely to be loaded with viruses!
A common type of phishing email are ones with attachments infected with viruses.
These might be disguised as an invoice, a delivery note, or something else that is designed to encourage you to open it.
The file may even have been crafted to look legitimate when you open them so as to minimise suspicion - just because they open normally & look like the document they're claiming to be doesn't mean that they're innocent.
There's several indicators you could look for to confirm your suspicions of it being infected (eg if it asked you to "Enable Macros"), but to be safe it's best to just always assume the worst - and follow the steps below.
Check your PC for viruses
The most important step to take if you ever open one of these attachments is to check your PC for viruses - follow our guide to removing computer viruses. Make sure you follow the Recovery steps in Section 3 on that page too, including checking that your all your software is up to date.
It's also always a good idea to re-run the virus scan after a couple of days too - if the virus is new then it can take a couple of days for antivirus programs to catch up.
Delete the email and attachment
Keeping the email won't do any harm to your PC if you never touch it again, but to be really safe you should delete it (and any copy of the attachment if you saved it anywhere). This will help prevent any accidental opening of the attachment in the future, for example by a family member who stumbles across it.