First batch? Welcome to the hobby!!! Warning: It’s addictive!!!
Using a pre-hopped kit, like 1.7 kilos? If you want 5% ABV, I think you’ll want to use a full kilo of light extract and a half kilo of dextrose. Or just use cane sugar instead of dextrose, same results. It would also be wise to steep a few grams of crushed roasted barley, maybe 100-150 grams, something like that, while the water is heating up, but remove it before it reaches 75 C. This will add extra stouty flavor.
See my generic guidance for beginning brewers here, lots of good stuff for you to think about. Click to enlarge, then copy and paste into Word and print 'er out if you feel so inclined: