While I cannot write your entire eight-paragraph essay for you, I can suggest some main ideas that you might mention. Jefferson was important first because of the election of 1800, in which he narrowly defeated Aaron Burr for the Presidency. He was originally deadlocked with Burr, and the House of Representatives had to decide the election. This situation resulted in the 12th Amendment to the Constitution, ratified in 1804, which stated that electors would vote on separate ballots for the President and Vice-President (instead of voting for two people, as was formerly done, and having the winner be the President and the runner-up be the Vice-President).
Once in office, Jefferson began to reverse the Federalist fiscal policy that Adams and other Presidents had put into place. For example, he discontinued the excise tax on whiskey, which had been wildly unpopular among farmers, and he put into place a more agrarian fiscal policy that reduced the size of the federal government and reduced the deficit. He attempted to dismantle the Bank of the U.S., a central feature of the Federalist plan (started by Hamilton), but was unsuccessful doing so. He also pardoned several of the people who had been jailed under the Alien and Sedition Acts, passed under John Adams.
In a move that was somewhat contrary to his idea of reducing the size of the federal government, he bought the Louisiana Territory from France in 1803. The Louisiana Purchase doubled the size of the country, and Jefferson sent the explorers Lewis and Clark to investigate the purchase. Their expedition, from 1804 to 1806, uncovered knowledge of the natural world in the new territory as well as knowledge about the Native Americans who lived there.
In foreign affairs, Jefferson passed the Embargo Act of 1807 that made American exports to Britain and France illegal. The goal was to force Britain and France to respect American shipping rights during the Napoleonic Wars. The act unfortunately mainly hurt American manufacturing. In addition, Jefferson sent naval troops to fight the First Barbary War against the Barbary pirates off North Africa.
Jefferson's purchase of the Louisiana Territory had a lasting effect on the future of the U.S., as it vastly increased the complexity and size of the country. His intervention in foreign affairs, while not entirely successful, increased American power abroad over time. However, his attempts to keep the U.S. an agrarian society largely only caused sectional conflict between the North, which pursued a more industrial program, and the South, which wanted to stay agrarian. Agrarianism, in addition to the argument over slavery and states' rights, were causes of the Civil War. Jefferson's commitment to agrarianism and states' rights, as well as his continued commitment to maintaining his own slaves, were parts of his legacy that can be considered negative.