The gravity at the final volume is what matters. You can actually get a more accurate reading if you measure before topping off and then convert it mathematically, but you'll have to waste a bit of wort to do so (proportionally more sugars wasted since it's more concentrated at the time).
Assuming you're doing an extract kit beer, honestly you can just not bother taking the original gravity reading since as long as you do everything correctly it'll be correct, and more often than not the errors new brewers have with their gravity readings are with the way the reading is taken, and not with the actual gravity. However, taking an OG reading is common practice and it's certainly not harmful to get in the habit of doing it.
Although just to be clear, you can't tell ABV with just that first reading, not until it's fermented and you know where the final gravity ended up.
