What should I do right after I arrive in Bulgaria on a Type D visa?

Arriving on a Type D visa is the start, not the end, of the process. Soon after entry you need to apply to the Migration Directorate for the residence permit corresponding to your ground of stay, and complete any required registration of your local address.

Do not let the validity of your entry visa lapse before you have your residence permit application in. Treat the post-arrival steps as time-sensitive, keep all your supporting documents to hand, and follow the deadlines closely so you move from visa-holder to lawful resident without a gap in status.