Please take note that before Imam al Mahdi arises, there are some signs that have to happen and will happen.   We have heard of a lot of signs that have already sought our senses, but these are the ones that reveal to us the closeness of Imam al Mahdi's coming.
Before the Big Day of Judgment (Qiyamat al Qubra), there is going to be Qiyamat al Sughra before it.

Qiyamat al Sughra is when Sufyani will emerge in Palestine and will start a revolt in the Holy Month of Rajab.  It is start in Rajab and end in Safar (8 months).  He will conquer Egypt for four months, then he will conquer Tonesia, Algeria, Morocco, Jordan, and Syria.  Damascus will be his headquarters.

 Sufyan's army will have 70,000 people who are illegitimate children and Jewish soldiers.
 Another sign that will occur is the situation of two boys by the names of Hasan and Hussein who Sufyani will torture, while only at the tender age of ten. Sufyani will also destroy the Mosques of Kufa and then go and attack Medina.

 There will be a desert named Baida which is between Makka and Medina.  As 100,000 soldiers of Sufyani will be passing through it; the earth will eat them up, only leaving two survivors.  These two survivors will have the names Bashir and Nazir.  Bashir will go tell the Imam the good news and Nazi will tell the Imam  the bad news of this issue.

 The Imam will then escape from Medina to Makka.  He will be wearing Prophet Muhammad's (s.a.w.) turban, Prophet Musa's walking stick, Imam Ali's sword - Thulfiqar, and Prophet Sulaiman's (a.s.) ring until his 313 followers will be gathered there for him.  But do we know who these 313 helper will be.  Imam Ali (a.s.) said that when the 'Qa'em of the Family line of Prophet Muhammad (s.a.w.) will emerge, Allah is going to gather together the inhabitants of the East and West.  His companions will be from Kufa and the brave ones who would assist him would be from Syria.

 Another sign that has to happen is that a Comet will appear in Safar before the Thohor (which will be he eighth month of Sufyani's tyranny.  A very significant sign that has to happen is that the 23rd  of the Holy Month of Ramadhan must fall on Laylatul Jum'a and Laylatul Qadr.  On the Midnight of Thursday, Imam al Mahdi (a.s.) will enter the Holy Ka'bah wearing sandals, carrying a stick and a few goats in disguise, which would be a Friday, the 10th of Ashura.

 Imam al Mahdi will also have with him when he comes out Prophet Jesus (a.s.), Prophet Khothir (a.s.), Prophet Ilias (a.s.), Prophet Idris (a.s.) Angel Jibrael (a.s.), and  Angel Mika'eel (a.s.). His residence will be in the Najaf, in which his capitol will be.  His masjid will be Masjid al Sahla.

Most of us have heard of the Dajjal.  Well, to give us all an even deeper insight of what has to happen for the Imam to arise - the Dajjal has to come out.  And even more specific, the Dajjal will come out 18 days before the appearance of the Holy Imam.
 And for some additional advice, it is said that reciting Surat Al-Fatiha keeps away the Dajjal. Imam Jafar as Sadiq (a.s.) was asked as to when the Holy Imam will come out.  He (a.s.) said that he will not come out in an odd year.

 Muhammad ibn Hanafia said from, 'Mustadrak', "We were in the presence of Imam Ali (a.s.).  A person asked the Imam some question about the Mahdi.  The Imam (a.s.) replied:  "Alas!  and he repeated this seven times and then said:  Mahdi will emerge at the end of time when those who would call out the name of Allah would be killed."

 Thereafter when this happens, Allah (s.w.t.) will gather together a group whose sagacity and agility will be like a cloud and He will make their hearts intimate towards each other.  they will neither fear anyone nor will they flee.   Their number will be equal to the number of the companions of 'Badr'.  Their number will be equal to the number of companions who crossed the river with Saul...

 It  is also narrated from the Prophet Muhammad (s.a.w.) said:  "During Mahdi's era, my nation will enjoy such bounties which they have never enjoyed before the like of it.  The sky would descend rain upon them while the earth would not neglect  anything from its vegetation but that it would expel them out.  Wealth at that time would not have any value such that a man will rise and say:  ' O Mahdi grant me wealth'.  And Imam al-Mahdi will reply and say:  Take... "

 If you've ever wondered why Imam Muhammad was given the nickname Imam al Mahdi is because of the following:  Abu Amro Mada'eni in his 'Sunan' has said: 'The reason why Mahdi has been called as Mahdi is because he will be directed toward the mountain of Syria and will take out the books of Torah from it.  He will argue and rationalize with the Jews by means of  Torah and a congregation of Jews will surrender to him.'