What to do if I have a tooth post in my mouth and since the MRI is magnetic I wasn't sure if that will effect it or not?

Not. Tooth posts are almost never made of iron or steel which are magnetic. Ti or plastics are not affected by MRI proceedures.
Post in tooth. Most materials used in posts are non-magnetic, Stainless steel and titanium.
No problems. Internal post for the tooth will not affect the MRI. CT images being more affected by dental alloys and other metals.