I was skinny for years and years, I eventually broke the cycle of lifting for three months, fuck all happening and giving up again for 6 months that had lasted for over a decade.
The way I did that was by keeping it super simple. Working on abs is fine but they're a small group, isolating at this point isn't the best use of your time. Forget the sit ups, shrugs, curls etc etc.
Do this:
https://stronglifts.com/apps/
Get the app, put in the details, keep it updated, track your progress.
You can do this and nothing but this for at least a year before you start to plateau, maybe even two years at a stretch, don't start reading anything about deloading, split workouts, any of that stuff for a long long time. Fact is that if you get to the stage where that matters you are already looking better than 99% of guys out there and lets face it, at this point you probably just want to look "average" and you can hit that in three months no problem. In a year with this everyone will notice that you are someone who 'lifts' and you'll be twice as strong as you are now.
Next up diet, at least 60% of the battle. Good news is that you get to eat all the shit that you want! If you want to be super healthy on the inside then sure clean bulk, but your muscles don't care, they can make as much new tissue off a mcdonalds patty as they can a piece of wagyu beef. Just remember to track what you eat and make sure you get enough protein in (aim for 1g of protein for each 1lb of your bodyweight), where the rest of the calories come from is your call.
Work out your resting calorie needs (plenty of calculators out there), throw an extra 200 calories a day on and track your weight gain. Aim for about 1-2lbs a week weight gain. If you're doing the progressive workouts as listed above a good chunk of that will be muscle (some will also be fat, that's almost unavoidable but try your hardest not to worry about that at this stage, it is a SHIT TON easier to lose fat than it is to gain muscle so just focus on this for now). If you're gaining significantly more than 2lbs a week though back off the calories a bit, you're going in too hard. If you're gaining less then add more calories in.
You're going to need to supplement with protein shakes to get in your daily protein requirements. They're not so bad once you find a flavour you like. If you find you're struggling to get in the calories then get a protein powder with plenty of dextrose in, usually called 'mass gainers' or something like that.
Last up it might be worth hitting the creatine. I'm yet to hear of anyone having a negative reaction to this and it does definitely work so why not.