Advice on 3 week old kittens

My kittens are now 3 weeks old, as this is the first litter I've ever cared for and I need some
Help my questions are when should I introduce the kittens to the litter box and what is the best litter to use with kittens also when should I be introducing them to wet food and what kind off food is best for there first introduction ??
Thank you in advance xx

Is Mummy's litter box nearby? As they will watch her using it. You need non-clumping litter as they will try and eat the litter and clumping litter will swell up in their tummies. If they have accidents anywhere then you can pick up solids and place some of it in the tray so that the smell is in the box. You can also soak kitchen roll with any urine you find and do the same thing.
You could try them on a pâté type wet food when they are 4/5 weeks old, try loosening it up with some warm water to help them as they will lick it at first. If Mummy eats nearby then they might try her food.

Keep them in a small area for now, preferably one thats easily cleaned.  As above, use non-clumping litter and low trays.  I put an extra one out as the mother cats I've fostered have all used a hooded litter tray with a fairly high entrance.  They will copy her - in my experience they are desparate to use her larger tray!  The same with food.  Feed her in the area where the kittens are and they will start copying her when they are ready.  It can start with paddling and swimming in her food and be very messy, and if she has a lot of milk and nurses them freely they might be 6 weeks or so before you see them eating food.

Whatever else you do, don't let the mother out until she is spayed.  She can easily get pregnant again while she is nursing her kittens.