I also personally have very limited knowledge about Mahdi (A.S) and probably need to read more.
I googled some Ahadith about this which are as follows:
Ali Ibn Abi Talib quoted the Prophet (peace and blessings be upon him) as saying: “Al-Mahdi is one of us, the clan of the Prophet (peace and blessings be upon him). Allah will reform him in one night.” (Reported by Imam Ahmad and Ibn Maqah).
2. There were three Hadiths narrated on the authority of Abu Sa`id Al-Kudri (may Allah be pleased with him). Such Hadiths were reported by Al-Hakim; and some of them were reported by Abu Dawud, At-Tirmidhi, Ibn Maqah, Imam Ahmad, and others. Of these Hadiths, At-Tirmidhi reported that the Prophet said: “Al-Mahdi is from my Ummah; he will be born and live to rule five or seven or nine years. (If) one goes to him and says: ‘Give me (a charity), he will fill one’s garment with what one needs.’”
3. Abu Dawud also reported a Hadith about Al-Mahdi that the Prophet (peace and blessings be upon him) said: “Al-Mahdi will be of my stock, and will have a broad forehead, a prominent nose. He will fill the earth with equity and justice as it was filled with oppression and tyranny, and he will rule for seven years.”
4. There is also the Hadith narrated by Thawban (may Allah be pleased with him) in which the Prophet (peace and blessings be upon him) said: “If you see the black banners emerging from Khurasan, seek to join their supporters even if creeping, because among them will be caliph Al-Mahdi.” That Hadith was reported by Al-Hakim and Ahmad. But the chains of narration of that hadith were all unauthentic, though some Hadith scholars rendered it sound in general.
Authenticity of Hadiths Pertaining to Al-Mahdi - IslamonLine.net - Ask The Scholar
And indeed something from Jamaat Ahmadiyya Website:
A few prophecies from the Books of Hadith are:
1. "Verily, God will appoint for this Ummah in the beginning of every century, those who will restore for it its faith." (Abu Da'ood, Kitabul Fitn)
1. "By the One who owns my soul, it is nigh, of a certainty, that the son of Mary will appear among you. He will be a judge, he will be just; so he will break the cross (refute Christian doctrines with reason and arguments--Commentary on Bukhari by Ainee) and kill the swine (a figurative speech denoting greedy, dirty and annoying person-- an abomination. Killing also is through spiritual means); and he will stop war." (Bukhari, Vol. 1, Babo Nozooli Isa)
1. "It is just possible that he who lives of you might meet Jesus the son of Mary who is the Mahdi and a just judge. He will break the cross and kill the swine..." (Masnad Ahmad Bin Hanbal, Vol. 2, p. 411)
1. "The Ummah can never die which has me at one end and the Messiah, son of Mary, at the other." (Ibn Maja, Bab Al-I'atisam Bis-Sunnah)
1. "There is no Mahdi but Isa." (Ibn Maja, Bab Shiddatuz Zaman)
1. "When 1,240 years will pass, God will raise Mahdi." (An-Najmus Saqib, Vol. 2, p. 209)
1. "What will be your condition when the son of Mary will descend among you and he will be your Imam from among yourselves." (Bukhari, Kitabul Anbiya, Bab Nazool Isa)
[The Advent of the Messiah and Mahdi](http://www.alislam.org/library/links/00000106.html)