Find the element (must be an input element with type="file" attribute) and send the path to the file.
WebElement fileInput = driver.findElement(By.id("uploadFile"));
fileInput.sendKeys("/path/to/file.jpg");
NOTE: If you're using a RemoteWebDriver, you will also have to set a file detector. The default is UselessFileDetector
WebElement fileInput = driver.findElement(By.id("uploadFile"));
driver.setFileDetector(new LocalFileDetector());
fileInput.sendKeys("/path/to/file.jpg");