Sorry - this question is very basic; within the app, have you logged in? On the bottom menu (Today, Food, Fitness, Weight, More) click More. Click Settings. Click Login.
It should sync itself automatically after that.
Welcome to SP! Take a look at the SparkDiet - it's a great way to start your lifestyle changes and helps you set realistic goals and points you to all the most used features so you won't get overwhelmed as a newbie. Here's a link: www.sparkpeople.com/resource/sparkdiet.asp
Those who think they have no time for a healthy lifestyle will sooner or later will have to find time for illness.