I'll try replying in brief with the use of many resources.
Although the exact time is known only to Allah, but Prophet SAW gave us some signs.
Imaam Ahmad narrated that Jaabir ibn ‘Abd-Allaah (may Allaah be
pleased with him) said: “The Messenger of Allaah (peace and blessings
of Allaah be upon him) said: ‘The Dajjaal will emerge at a time when
religious commitment is low and knowledge has decreased… Then ‘Eesa
ibn Maryam will descend just before dawn and will call people, saying,
‘O people, what is stopping you from coming out against this evil
liar?’ They will say, ‘This man is a jinn’, then they will set out.
When they reach ‘Eesa ibn Maryam, the time for prayer will come the
iqqamah will be given, and it will be said to him, ‘Go forward (to
lead the prayer), O Spirit of Allaah.’ He will say, ‘Let your imaam go
forward and lead you in prayer.’ When they have prayed fajr, they will
go out to meet him (the Dajjaal) and when they see the liar, he will
start to dissolve like salt in water. ‘Eesa will go to him and kill
him. Even the trees and rocks will call out, ‘O Spirit of Allaah, here
is a Jew!’ And none of those who followed him will be left, they will
all be killed.” (Hadeeth no. 14426).
Where will he appear?
Anas (may Allaah be pleased with him) said: “The Messenger of Allaah
(peace and blessings of Allaah be upon him) said: ‘The Dajjaal will
emerge from among the Jews of Isfahaan, and with him will be seventy
thousand Jews, wearing crowns.’” (Narrated by Ahmad, no. 12865).
He will emerge from Isfahan which is Persia, modern day Iran. However, another hadith states that he will emerge from Khurasan.
What would he look like?
Abu Hurayrah (may Allaah be pleased with him) said: “The Messenger of
Allaah (peace and blessings of Allaah be upon him) said: ‘… as for the
false messiah, he will be one-eyed, with a wide forehead and broad
upper chest, and he will be hunchbacked…’” (Narrated by Ahmad, no.
In another hadith, it says:
Hudhayfah (may Allaah be pleased with him) said: “The Messenger of
Allaah (peace and blessings of Allaah be upon him) said: ‘The Dajjaal
will be one-eyed, blind or defective in his left eye, with thick hair.
He will have with him a paradise and a hell, but his hell will be a
paradise and his paradise will be a hell.’” (Narrated by Muslim, no.
Moreover, according to another report:
“Written between his eyes will be ‘kaaf faa’ raa’.’” (Muslim, no.
5219) According to a report narrated by Hudhayfah, “This will be read
by every believer, literate or illiterate.” (Muslim, no. 5223).
Would he have some kind of powers?
What will he do?
The fitnah of the Dajjaal will be the greatest fitnah from the time Allaah created Adam until the Hour begins. This will be because of the mighty miracles that Allaah will create with him, which will dazzle people’s minds and amaze them.
In the hadeeth of Faatimah bint Qays (may Allaah be pleased with her),
it says: “(He will say:), ‘I almost have permission to emerge. Then I
will emerge and will travel throughout the earth for forty days, and I
shall not leave any town without entering it, apart from Makkah and
Madeenah, which I will be prevented from entering because every time I
try to enter, there will be an angel with an unsheathed sword in his
hand, preventing me from entering. On every gate there will be angels
defending them.’” (Narrated by Muslim, no. 5228)
In the hadeeth of al-Nawwaas ibn Sam’aan about the Dajjaal, it is
reported that the Sahaabah said: “O Messenger of Allaah, how long will
he stay on the earth?” He said, “Forty days: one day like a year, one
day like a month, one day like a week, and the rest of the days like
your days.” … They said, “How fast will he travel through the earth?”
He said, “Like the clouds when they are driven by the wind. He will
come to some people and call them, and they will believe in him and
respond to him. Then he will command the sky to rain, and the earth to
bring forth vegetation, and their cattle will come back to them in the
evening, with their humps very high, and their udders full of milk,
and their flanks stretched. Then he will come to another people and
will call them, but they will reject him. So he will leave them and
they will be stricken with famine, with none of their wealth in their
hands. He will pass by ruins and will say, ‘Bring forth your
treasure!’ – and the treasure will follow him like a swarm of bees.
Then he will call a man who is brimming with youth, and will strike
him with a sword and cut him in two, then (he will put the pieces
apart from one another) the distance of an archer from his target.
Then he will call him, and the young man will come forward smiling,
with his face shining.” (narrated by Muslim, 5228)
For more detailed information, watch this lecture by Dr. Bilal Philips on Dajjal: Sign of the Last Hour (https://www.youtube.com/watch?v=gno4sufkPnE)