You've probably heard the cry: "my dog sits just fine at home, but he won't do it here". This is normal. When you go to the park, there are smells to sniff, people to look at, perhaps even other dogs. The way around this is not to despair, but just to practice your sits in slightly more and more distracting environments. After doing it in the house, try the back yard. Or maybe the parking lot. Only after you have practiced sits in a variety of locations, move up to using the command when the dog wants something else. For example, when you and your dog answer the door, eventually you'll be able to tell your dog to sit so you can answer the door. But meeting a new person is high distraction, and so until you've gotten good at sits in easier settings, you might need to rely on a leash or other means to deal with the door. Progress can seem very slight from day to day, but if you keep practicing sits which your dog gets right (give them a chance to succeed), then you will find that little by little you can work up to more and more distracting situations.