Iran grants Azerbaijani citizens visa-free entry unilaterally. On an ordinary passport you may stay for up to 15 days. That is one of the shortest periods on the list, so plan the trip around it.
If you want to extend your stay or remain longer than 15 days, a visa must be arranged in advance. At the border you may be asked for a return ticket and an address of stay; queues at land crossings can grow depending on the season.
Tabriz is chosen for trade and short family visits, Mashhad for pilgrimage routes. Transport options include bus, private car and plane. Viza.az builds a route that fits the 15-day limit and compares the transport options.