Dumb question about Tricare - after military

My husband is set to retire next year after 20 years. He'll be 38.

We have a baby. Currently, we use a military hospital (Fort Belvoir). After he retires, I'd like to buy the Tricare insurance for baby and me. How does this work -- meaning, should I search for a private doctor?

Or will we still be able to continue using Fort Belvoir? Or how do we pay the co-pays and how can we pay for the appointments?