I need a friendly dog as well have quality of guarding (security purpose also) what the best dog breed you can suggest for beginners?

I am a female of age 20 who is doing my medicine and quite busy planning my career. Both my parents are also doctors who are more busier than me. So firstly we don't have that time to take care of  https://www.dogspot.in/like to take the dog to pet doctor very frequently. It should easily get adjusted in our big house and also acts as a security purpose for our big house. We don't have any knowledge about how to raise a dog in a proper way and I think it's better to have a small puppy so that we can train it better from the beginning. So kindly please suggest me a breed of dog which is very easy to bring up and friendly one. It should not harm the strangers.