Start with the business associate agreement
Any vendor whose staff can reach protected health information is a business associate, and that includes an IT provider with administrative access to your systems. The agreement is not a formality to be handled at signature.
Ask who signs it, whether their subcontractors are covered, how breach notification timelines are stated, and what happens to your data and documentation when the relationship ends. A partner who cannot answer the offboarding question has not thought about the part that matters most to you.