When walking a dog....?

I just got my first puppy and she's 4 months now but I'm not really sure about walking them. When they sniff the grass and stuff do I slow down and let her sniff or do I pull and keep going? And sometimes she's going so fast and I have a harness on her but she still wants to go fast how do I slow her down? yea I know it's not rocket science but my sister has a German Shepherd that is crazy and will choke himself while walking with a harness and I don't want mine to turn like that lol