The correct answer to this open question is the following.
Although you did not mention the time in the history of this event, we assume you are talking about the Lewis & Clark Expedition of August 1803.
The three things Jefferson might have looked for when choosing the leaders for the expedition were to be adventurous, have the experience as explorers, and determination to never quit.
After the Louisiana Purchase of 1803, when the Federal US government bought the Louisiana Territory to the government of France, US president Thomas Jefferson ordered an expedition to explore the west part of the territory. He named Merriweather Lewis as the leader of the Expedition, and he was assisted by William Clark.
The Lewis & Clark Expedition ended in September 1806 and it laid the groundwork for future US citizens to begin colonizing the "West" of the United States.