I highly doubt Dante's Inferno requires a full-blown introduction--if you're reading along, I'll leave that to your chosen translator (mine happens to be Anthony Esolen)--but I think a few notes on Canto One are in order. (I'll be expanding on some of Esolen's notes. And, since this is the most allegorical canto in the Inferno, it's worth looking at in a little more depth.)

• "Midway upon the journey of our life ...": Dante is about thirty-five years old--middle-aged--when he descends into hell. (Furthermore, this line fixes the year at 1300; Dante, born in 1265, would have been thirty-five in 1300.) Also, since the Inferno is theological, consider Psalm 90:10: "The days of our years are threescore years and ten"--or seventy years, in modern parlance.
• "... I found myself in a dark wilderness ...": This line is interesting: as allegory, it represents sin, confusion, and corruption. But the sin and corruption of what? The papacy, certainly: in Canto Nine, Dante places Pope Nicholas III in the eighth circle of hell--specifically, in the pouch reserved for Simonists. The dark wood also represents corruption of politics: Dante began work on his Commedia while in exile, after the collapse of his political party, the White Guelphs. With the rival party, the Black Guelphs, in power, Dante was tried in absentia for crimes such as embezzlement and graft, and threatened with death by fire if he ever returned to Florence. If we examine Dante's Convivio, and remember Dante's age, the second line also refers to the corruption of youth: "So the youth, who enters into the wandering woods of this life, would never know the right way to go, were it not shown to him by his elders." (This line from the Convivio also explains Virgil's presence in both the Inferno and the Purgatorio.) I've always been struck by this line for one other reason: Dante places himself as both the subject and object of the verb, which enhances the personal nature of his own sin. He finds himself lost.
• "... the sun was mounting higher with those stars ...": Scholars agree the Inferno begins on April 7, 1300--Maundy Thursday, or the day before Good Friday. (See Canto Twenty-one, lines 112 to 114.) Canto One also gives a good indication of how fast-paced the poem really is. Dante spends all of Maundy Thursday with the three beasts. By the time he and Virgil enter ante-hell (Canto Two), it's already Good Friday.
• "... that just son of Anchises ...": This is where Virgil's Aeneid comes in handy: Anchises' son was Aeneas, the hero of the Aeneid and the founder of Rome.
• "... until the Greyhound comes ...": Dante is alluding, vaguely, to a savior, though almost certainly not Christ. Scholars aren't sure who or what the Greyhound symbolizes, but one theory is that it's Cangrande della Scala, one of Dante's benefactors during the poet's exile. (Cane grande means great dog.) Depending on the translation, this could imply a number of meanings; as Esolen notes, "The Greyhound's victory is to be seen in apocalyptic terms, for the beasts fornications recall those of the whore of Babylon ..."; Robert Pinsky's translation is quieter: "Many are the beasts she mates: there will be more / Until the Hound comes ..."; he makes no mention of the "apocalyptic" nature of the Hound. (Incidentally, while I appreciate Pinsky's subtle Inferno translation, I don't recommend it for Dante initiates. Esolen, a professor, brings out Dante's meaning; Pinsky, a poet, brings out Dante's music--his terza rima. Translators will inevitably make sacrifices--music or meaning. Unless you read Italian, you can't have it both ways.)
• "... so I may see Saint Peter's gate ...": When reading Dante, you'll have to discard your Sunday school notions of heaven. In the Commedia, salvation begins at purgatory. Contrary to popular (evangelical) belief, Saint Peter's gate isn't located in paradise; it's in purgatory. (The Inferno is a Christian work, but with a distinctly Catholic theology.) Consider Canto Nine of Dante's Purgatorio (translated by Jean Hollander and Robert Hollander):

'From Peter do I hold them, and his instruction was
to err in opening rather than in keeping locked,
if but the soul fell prostrate at my feet.'


Then he pushed one door of the sacred portal open,
saying, 'Enter, but I warn you
he who looks back must then return outside.'

Next: Canto Two. This is where you'll need the Aeneid.