This is a good good question, but partially easy to answer. Before 966, the land of Polans was of no interest to Romans, as it did not lie at that time on any major trade route. Nothing could be recorded in Poland before 966, until the first clergy arived, with ability to read and write - obviously in Latin. Earlier sources are only of the foreign origin, such a chronicle of Ibrahim ibn Jakub (Ibrâhîm ibn Ya`qûb) (961-962), who visited Poland during Mieszko I reign.
The further explanation is taken from a paper in Polish The origin of Poland in a new light by Prof. dr hab. Tomasz Jasiński, Uniwersytet im. Adama Mickiewicza, Instytut Historii, Poznań .
First fuller mentions of tribes from or near Poland come from "Bavarian Geographer", a 9th c. source from monastery of St. Emmeram in Regensburg (Czech Řezno, Polish Ratyzbona), Bawaria. It lists tribes, including number of strongholds or regions. Some of such names are very readable and recognizable:
18 - Goplanie (Kuiavian region), 33 - Lędzianie (East of Lesser Poland), 48 - Wiślanie (Lessar Poland) , 49 - Ślężanie (Wrocław region), 51 - Dziadoszanie (near Głogów); 57 - Opolanie and 58 - Gołęszycy (near Racibórz).
But that list does not mention Polanie, which have puzzled the historians for decades. The answer apparently was found recently, with the help of natural sciences.
The oldest mention about Mieszko I was written about year 1000, by a Saxon chronicler Widukind from bendictine monastery Corvey near Höxter. It mentions battles between Mieszko I and graf Wichman in year 963.Gero returned Wichman back to the barbarians, from where he got him. He, being welcomed by them, destroyed the barbarians living further away during his many attacks. He defeated Prince Mieszko, a ruler of Slavs who call themselves Licicaviki, two times, killed his brother and snatched from him a great booty.
More information about beginning of Poland comes from dendrochronology.
[Example: Now we know for sure that most of the trees used to built Biskupin stronghold were cut down between 738 and 737 BC. This has nothing to do with the beginning of Poland; it just illustrates accuracy of the method]
Based on 1999 explorations we now know that most of the strongholds in Greater Poland are younger than previously thought. Gniezno stronghold was built in 940 (not in 800s), so it is younger than Mieszko I himself, probably born in 935.
Today we know that the beginnings of Giecz, Moraczewo and probably Poznań are dated at around 850s. Several strongholds were being erected around Gniezno Uplands, incrementally so to speak: A powerfull stronghold in Grzybów about 915-922 (many times improved upon and expaned after that), a stronghold at Ląd just after 926, and at Bnin - before 934. That was an evolutional process.
Situation changed in between 930-940. New strongholds were built in 940 in Bnin, Giecz, Ląd. In 939-942 Grzybów was expanded for the last time. The second stronghold in Poznań was very probably built in about 940.
So five new strongholds were built in 940, protected from north, west and south by the rivers Noteć, Wełna and Warta. All oak trees were cut down from the Gniezno Upland. The hypothesis is that the reason for building so many strongholds in such a short time must have been caused by a sense of a great danger. What kind of danger? In 940, German King Otto I crushed the great uprising of the Elbe Slavs. They mutinied in 936, in aliance with Czechs, at the time of intronisation of Otto I.
That felt like a great danger: "we could be next", but not only coming from Saxony but also from Bohemia. This is why Mieszko decided to join the Christianity: to neutralize both and to knock out the argument of necessity to convert Polish pagans by sword. But before he did so Mieszko I expanded enough to assure safe border for his new kingdom.
The dendrochronology proves that incorporation of south and south-west Great Poland, as well Sieradz and £ęczyca - and probably Kuiawy, Mazovia and Gdansk Pomerania - was done in 950s. In addition, new strongholds were built, which replaced the old ones, destroyed during previous wars.
When Poland was first mentioned in 960s in first written documents Mieszko I fought with Welets and their allies Wolinians.
Poland about 940