You want to look for a nice canned food (much healthier than dry for him). Whatever food you choose you want to look for something that's real meat (no byproducts or meals) and something grain free. That'll give him a good healthy start in life.
Call the vet first thing and take him for his shots and a checkup and make an appointment to have him neutered. Perhaps you can offer to pick up mom in laws cat and have her spayed at the same time so she doesn't get preggers again. Don't let the vet talk you into expensive kitten food that he just happens to sell there.
Just be sure to show him where the litterbox is and since he's a baby he might not be able to 'wait' to get that far. You may want to put a box on each side of the house at first so he can get there quick. If he's REALLY tiny he may need a brick or a book to help him get over the side of the litterbox.
A little baby, I would put him in his own room when you're gone with litter box, food, water and toys (but that's just my opinion). This would be a good place to put him when you first bring him home so he's not overwhelmed by a giant house and being all by himself. A teddy bear to cuddle with on a cat bed or a little pile of blankets with maybe a ticking clock to simulate a heartbeat. As he grows up and shows interest in exploring you can let him.
Buy him scratching posts with sisal and maybe a cardboard one as well and when you see him using it, praise him to the heavens. I agree with Morquinn, I'd put a blanket on the couch for now because when he's scrambling to jump up onto it when he's little he may dig his claws in to help himself up. Also, when you have him at the vet ask them to show you how to trim his nails and ask him to let you trim a few with his supervision so you get the hang of it. This will help with accidental scratching and your human baby and the younger he gets used to it the easier it will be when he's older. Also I would do as Morquinn said and if he's scratching at the couch give him a "NO" and carry him to the scratching post and put his paws on it praising him. (I only had to do that with MowMOw twice and he never touched my couches again).
Toys will be important. He's going to have ENDLESS energy and a tired kitten is a well behaved kitten
. Remember to never let him 'play' with hands/feet. It's cute when he's little but it'll hurt and leave scratches when he's older. When he uses his mouth/claws, say "OUCH" and put him down and walk away. Play time is over.
As for peeing on couches, don't borrow trouble. If he's taught properly as a baby chances are good he'll never do it and if he suddenly starts there is usually a medical reason.