Please please please please please for the love of god do not buy a puppy from a pet store!!!!!!!!!!!!!!!!!!!!!!!
The only way to go is to buy from either a registered breeder who has done all the appropriate health checks etc. or go for a rescue. Depends on your price range, purebred dogs are between $300 - $1000 rescue dogs from $0 - $200. As has been said above buying from a pet store is only supporting a discusting industry that causes countless dogs to suffer and many more being pts because of pet overpopulation.
One thing you'll need to realise is that having a puppy is just like having a baby in the house... that doesnt wear nappies and has needle like teeth!
Be prepared to have your patience tested. Thats the great thing about rescues is you dont necessarily have to get a puppy, you can find a dog that fits all your requirements (good with cats, kids etc.) and has already passed all those annoying puppy stages. I mean when i got Tyson he was 16 weeks old so already passed the no brainer puppy stage where they dont seem to think before they act, his toilet and lead training was pretty much already done for me and he already knew how to sit etc. but at the same time he's still a baby and i can still get to watch him grow.
The thing you also need to be careful of with cross breeds is as much as people like to claim it, you have no idea how big they will get or what exactly they are REALY crossed with. My dog Pixie was apparently a maltese x cavalier she ended up growing up to be 100% jack russel.
No exactly the best breed of dog to have in a house full of small animals as pets.
Okay as for toilet training. I realy recommend you research into 'crate training' that makes things sooo much easier. The key is to not allow the puppy a chance to make a mistake. He's going to have to be taken outside every hour (especially after eating and sleeping) and at night time you should either crate him and take him outside whenever he cries so he can go toilet or have him sleep in a puppy play pen set up next to your bed where the ground is covered in news paper and he has his bed and food. (I can post a pic if you like)
The best thing to do would be to reasearch into the breed of dog you are getting, that gives you the best heads up on what to expect and whether the dog will suit your family. If you want you can list your expectaions and wants in a dog and i can tell you what breeds would suit? Of course shelter dogs are realy the easiest way to find a dog will suit your home since their temperment has already been assesed and you know exactly what you're getting (especially if they're already fully grown).