According to Adam of Bremen, the Norse temple at Uppsala was surrounded by groves in which various creatures were sacrificed in groups of 9. The human sacrifices were supposedly criminals. Bear in mind, however, that there is no concrete archaeological evidence for any temple complex, let alone such a temple as Adam describes, and that Adam of Bremen also says that Russia is inhabited by Amazons, Blemmyes and Cynocephali, so anything he says has to be taken with a massive pinch of salt. It's important to remember that practically all of our written sources for Scandinavia are later Christian histories, or written with an aim towards Scandinavian conversion, so actively seek to portray the old Scandinavian religions as bloodthirsty and barbaric, that people would be better off without. There are no records of sacrifices from, say, Iceland, or the Danelaw in England.
First of all, we're not entirely certain that humans were actually sacrificed at Gamla Uppsala.
The only source we have for it is Adam of Bremen, a German monk and missionary who did not himself visit Gamla Uppsala but instead repeated what he was told by local christians. It is possible that the sacrifices are exhaggerations by christian chroniclers trying to make the pagans out as worse than christians. It is of course also possible that sacrifice did take place at Uppsala as Adam of Bremen describes it - one person being thrown into a springwell, and if he did not emerge again, people's wishes would be fulfilled and nine people being sacrificed over a time period of nine days and hung out in a glade.
Other blot described by Snorri Sturlasson in Heimskringla and Ibn Fadhlan includes the blod of animals being spattered or painted on special poles, but no human sacrifice.
As for the original question, there's no information, as far as I know, beyond Adam of Bremen's of human sacrifice at Gamla Uppsala, and he does not go into detail on how those sacrificed were chosen.
According to Christopher Abram, Maria Lundborg and others (see below for refs), one of the main difficulties in answering your question is the cultural vandalism the early Christians inflicted on Scandinavia. The sources of Norse mythology and religious practice have largely been destroyed and what we have is pieced together from a multi-disciplinary examination of the remaining Skaldic/Eddic poetic works and interpretation of archaeology and pictorial sources. Unfortunately what has been passed down to us has often been intentionally “edited” to present a view that may reflect what the editors (in the loosest sense) wanted us to understand rather than actual practice/belief. We may never know what motivated the practice of human sacrifice in Scandinavia or indeed whether it actually ever took place. All we can do is guess. For a general account see Myths of the Pagan North by Abram or for more detailed academic work see Old Norse religion in long-term perspectives edited by Ander Andren & others.
The second part of your question may be easier to answer generally by reference to the practice of human sacrifice in other cultures close temporally and geographically to Scandinavia. There are good sources documenting human sacrifice in ancient Crete, Tyre, its off-shoot Carthage and later in the Roman Empire. The differing practices all point to the use of human sacrifice as the ultimate reaffirmation of civil power. Ancient religion and politics are indivisible. At times of extreme existential crisis – when the barbarians are at the gate – the human sacrifice is a method by which the elites could not only demonstrate their power over society but by that demonstration force those under them to acquiesce in it. The subjects of the sacrifice would be chose from the highborn or their children. It would be considered an honour to give up one’s own (of one’s children’s lives). Put simply, a society willing to kill its own children is reaffirming a strong message for those within it. It is the ultimate pep-rally.