Can you be admitted into hospital for depression even if you're not yet taking medication?

That depends: Unless you are a risk to self or others, psychotic or unable to function - it is unlikely that you would be admitted. In these types of situations, admission may be appropriate even if the person is not on psychiatric medications.

Of course: If someone is suicidal, psychotic, or unable to care for self, s/he might be admitted even having had no prior psychiatric treatment. Many people avoid seeing psychiatrists until they can avoid it no longer because they're so sick. Managed care can make getting inpatient beds hard sometimes, though. As a psychiatrist i've often been on the phone "in queue" for hours with insurance companies.