Step One: From Outlook, under the Delete group, click the Junk button.
Step Two: Select Junk E-mail Options...
Step Three: By default No Automatic Filtering is selected. Select Low or High as desired.
Step Four: Click OK.
Be aware that High will, as it warns, catch some legitimate email and move it to Junk Email.
Items in Junk Email are auto deleted after 14 days. Please check the contents of Junk Email regularly to avoid missing any falsely flagged email.