The guitar was my uncle's before it was mine, left in our garage in its original case after he moved to Arizona for a job, a shipping label from a store that closed years ago still stuck to the side. Nobody had touched it in the meantime except to move boxes around it.
Your first day
Tune the guitar with a free phone tuner app first, then learn to hold down a single G chord. Your middle finger goes on the thickest string at the third fret (frets are the metal bars along the neck, counted from the tuning pegs), your index finger on the next string down at the second fret, and your ring finger on the thinnest string at the third fret. Place each finger one at a time, checking that string rings clean before adding the next. Once all three fingers are down and every string rings, strum slowly downward, all six strings, four times. Lift your fingers all the way off the strings instead of just loosening your grip, then reset the chord from scratch and do it again.
Most beginners don't press hard enough right behind the fret, which produces a muted buzz they often mistake for their fingers being in the wrong spot entirely. Pressing just behind the metal fret, not on top of it, usually clears a buzzing string without moving the finger at all.
The thing most people miss
The fingertip pain that made me doubt my technique was just callus forming, and it goes away within two or three weeks of playing regardless of how good or bad your technique is. Sharp pain in a wrist or joint is different, and means stop and check your posture.
I'd even started icing my fingertips between sessions for a while, which turned out to be unnecessary. Once I stopped babying my fingers between sessions and just kept playing through the mild soreness, the callus formed faster than I expected, and the chord itself stopped being the hard part.
What not to buy
Don't buy a capo or extra picks before a clip-on tuner. An out-of-tune guitar makes even a correctly fingered chord sound wrong, and that's the actual first mistake beginners run into, not a missing accessory.
The switch from G to C stopped needing my attention around the same time I stopped noticing I was making it, hands moving on their own mid-conversation with someone in the room. The G to C to D progression is what an enormous number of songs turn out to need first.