Well, you're assuming that the government does, in fact, work. Ok, couldn't resist; here's the basic explanation.
First thing is to note that there is a federal government and then the state governments, and generally the state governments are arranged more or less like the federal government. Each state has its own Constitution and their government plan may not quite be like the other states, but they're usually like the federal government - just substitute "Legislature" for "Congress" and "Governor" for "President" when discussing the state level. Generally, the big things the state governments do include basic criminal law (laws against murder, stealing, etc), building roads, running the school system, and local sorts of things, while federal powers include decisions involving business that crosses state lines, war, and other national issues.
Congress's main job is to write the laws, as well as declare war and approve treaties. There are two sections, the House of Representatives and the Senate. The main differences are that the House is elected every two years and Senators serve six year terms with 1/3 up for election every two years, that there are two Senators per state but the number of Representatives is pegged to the state's population, and the Senate is the only one that needs to approve treaties.
The President approves the laws, and he's also in charge of the agencies that carry out the laws and Commander in Chief of the military. He appoints the head of just about anything with "Department," "Administration," or "Agency" in its name. For example, when Congress passed the Pure Food and Drug Act requiring that food be safe, correctly labeled, etc, this called for a Food and Drug Administration to check to be sure food and drug companies were obeying, which reports to the President.
The Supreme Court is the top court - there are a few kinds of disputes it hears directly, but it's usually for appeals from lower courts. They can sometimes declare a law unconstitutional or in conflict with other laws, depending on the circumstances.