I am a landlord and I need the apartment back. How do I tell the tenant that they must leave? How much time do I have to give them? Do I need special forms.

You send a letter, receipt requested, using the same frame under which they pay rent. If they pay rent on the first, for example, you must notify them before the first so they have a full 30 days to move. For example, you would give notice before July 1st that you want them out on August 1st.

In certain States you cannot simply evict. You must have a specific reason and those reasons can be limited. What State?

If you serve them with this notice and they don't move, then you have to do a full eviction proceeding.

Laws vary per state. Some states require a notice of one rental period. Others a straight 30 days, some even less than 30 days.

Also the reason for the termionation of the tenancy may have a bearing. Without knowing where you are, its hard to give a definitive answer.