keep an eye on them-- kitten abandonment does happen, but its also possible that the mom left them there as she went to do something.
kittens have different needs at different ages, if they're really small, one to 3 weeks, gather them up and bring them inside if it looks like mama isnt coming back for them. get some kitten milk replacement and a syringe/bottle from the pet store-- employees can help you pick out exactly what you might need. cow milk can be used as a stopgap, but kmr healthiest for them. Also get a box and fill it with old towels or blankets, and keep it in a quiet part of the house. this might sound wierd, but you also will have to rub a cotton ball on their butt area-- kittens can't go to the bathroom on their own until about 3 weeks in.
if they're old enough to run away when they see you, start putting wet cat food outside around the same time every day-- this is safer if their digestion is still young. stay outside a little yourself and use nonthreatening cat body language to show that you're trustworthy. this means little direct eye contact, not looking big, blinking and winking at each other, ect. if they like you, it will be easy enough to get them inside, where it is much safer.
For preparing the house, in either case have some reasonable kitten proofing measures (no crystal goblets in easily push able places) and a quiet place for the cat to escape if it gets overwhelmed.
googling kitten rescue will get you a lot of advice from vet offices, and they will need to see a vet eventually for shots and neutering. and they really need neutering, they can have their own litter as young as 4 months old!
i hope they turn out well, and that you enjoy the time together if you keep them!