You don't say what food your dog is on right now but if it is a high quality product then I would say continue with that - there is no need to change. If it is not a particularly high rated product then supplement with home cooked food - e.g. meat, fish, eggs, sweet potato, brown rice and cooked vegetables. Good quality protein is important. You will need to give more in the latter part of the pregnancy.
Once the pups are born you will need to give more food dependent upon the size of the litter. I have always supplemented the normal food with home cooked. As above, choose good quality protein - scrambled egg, chicken, minced beef, fish, sardines. If your bitch is not lactose intolerant, you can supplement with goats milk - it is easier to digest than cows milk. Calcium is important for the lactating bitch so look for food that contains this.
Regarding weaning the puppies there is a wide variety of choices. I tend to start with warmed goats milk to get them lapping then thicken it slightly with baby rice. After that I have used lightly cooked minced beef but there is no reason why you should not go onto a commercial food. Choose a good quality dry or wet food (use the Dog Food Directory to source something suitable). If using dry, you will need to soak the kibble for about half an hour before giving it in order to make it crumbly and easy for the pups to eat. If you need help in using the Dog Food Directory please ask.
If you have never bred before, read up as much as you can. The Book of the Bitch
might be helpful to you.