What are some definite signs of Imam Mahdi? Can you please tell them chronologically.

There are 5 inevitable signs, however these are directly associated with immediate appearance of the Imam (as), so that there is no excuse left for anyone on the face of the earth, to say: They did not know the Imam, or have any doubt about him.
1. The Sufyani : He will lead the dark armies, he may be what has been referred to as the Antichrist or the Dajjal (the imposter).
2. The Yamani : He is the harbinger of the coming of the Imam, a righteous man and a leader.
3. The sinking of the desert: an army of the Sufyani will sink into the desert and perish.
4. The killing of The Pure Soul: a Righteous man who is a descendant of the second imam (as) will be killed next to the Kaaba in Masjid al Haram.
5. The call in the skies: a call in the heavens that everyone will hear no matter where they are, that will let everyone know that the imam (as) has appeared.