Thorin had no realistic plan at the outset but that didn't seem to matter to him. He was the rightful King Under The Mountain and he was going to reclaim his kingdom or die trying. Warriors do not fear death, and kings like Thorin cannot bear to live as vagabonds exiled from their kingdoms.
He started out with what was familiar. He knew how to wage war, and he had loyal subjects who would call him King and would spend their lives to restore the glory of Erebor. But this idea was not really a plan, and it vastly underestimated the enemy.
Notice how in chapter 1 of The Hobbit there was really no discussion of how Smaug would be killed or expelled. They gathered, ate, and sang songs about the great kingdom that was lost and would be regained. A motivational session. That was it. Bilbo, and we the readers, are left to assume that they know what to do and it will be as easy as they seem to believe.
Gandalf provided all the actual early planning, and that wasn't much. Gandalf knew there was no chance that even thirteen great dwarvish warriors could just march into the mountain and kill or evict the dragon. Gandalf somehow knew that a burglar was necessary on this adventure, and that Bilbo of all people must be that burglar. Gandalf persuaded Thorin to seek refuge in Rivendell. Without Elrond, the means to enter through the side gate would have remained unknown.
Through their brushes with death at the hands of the trolls, the goblins, the wargs, and helpful encounters with Elrond, the Eagles, and Beorn, Thorin and the entire party came to rely more and more on Gandalf for guidance, assistance and survival. Notice how upset they were with Gandalf when he suddenly rode off without them at the edge of Mirkwood! They kept going, but this revealed just how dependent they were on Gandalf by that point. Thorin's plan by that point basically was:
- Get to the Lonely Mountain
- Gandalf "does his thing"
- No more dragon!
When they actually arrived at the Mountain and Gandalf still was not with them, Thorin didn't want to admit that after all this he had no real plan. So he ... sent in the burglar. "Go burgle something!" After all, Gandalf insisted this hobbit was essential to the company, right?
Finally, after the dragon was aroused, and they all realized exactly how outclassed they were, all they could do was huddle inside the secret mountainside entrance and wait for something to happen. Thorin with them. Bard finally took care of business and led the people of Dale to start the disaster recovery effort brought on by the recklessness of Thorin Oakenshield, who now had to either pay reparation or be taught a lesson.
Thorin had no realistic plan for defeating the dragon at any point. All he had was dwarvish stubbornness, kingly valor, a Company, a Wizard, and a Burglar. And that was all he needed.