I would look for a girl who looks healthy, has a happy temperament, and is in the 25 to 50ish pound range.
You need to get a good book to read on the subject right away. I would suggest "Successful Dog Breeding" by Bonnie Wilcox and Cris Walkowicz if you can only afford one.
If you can afford several, I also like very much the one by Dr. Phyllis Holst, DVM, "Canine Reproduction, A Breeder's Guide". It is a little techinical, but very detailed.
And I also like "The Joy Of Breeding Your Own Show Dog" by Ann Serranne. It does have quite a bit of breeding theory in it, but also a great deal of very good technical information about whelping and raising of puppies.
Most of the time, the mom dog does not need a lot of help. Most of the time it is best to let her clean the puppy and cut the cord, etc. If I have to break a cord, I use my thumbnails to crush the cord and break it. Birth is not a sanitary process, and this is no less sanitary than the normal way of the bitch doing it with her jaws.
If she does not immediately attend to a puppy, then you just take a towel and hold the puppy with head facing downward while you rapidly and briskly clean it off with the towel. Then you get another dry towel, and while holding the pup head down, you show it butt first to the mom, who will usually start licking it. Let her clean him for a minute. It is important that the puppy has his first bowel movement at birth or shortly after to clear his intestines and make him able to properly digest milk. So you want to make sure that happens, and then I weigh the pup, put an ID ribbon on it if needed. Then the pup is put on to nurse while I write down the sex, what order it is in birth, how much it weighed, what its ID ribbon is, or if I am identifying it by markings, I will note that. Then I just make sure that all puppies are nursing well, and wait for the next pup.
Once they are all born, I weigh twice a day for the first week, morning and bedtime. I want to see puppies steadily gaining. If someone is slow or not gaining by day 2, then I start making sure this pup nurses first and gets the best teat. If necessary I will supplement, but I prefer slower puppies get the mom's milk, and if necessary, I will bottle feed the biggest strongest ones, and leave the mom for the others to nurse out first.
By week 3, you will start some thick gruel feedings, and by week 6 mine are eating pretty much what the big dogs do, just in a smaller quantity, with more grains.
You should plan on keeping puppies until 8 weeks, or they should stay together that long, not sure what age the shelter would take them all back?