Good question! Until you are signed off for unlimited cross-country trips by your instructor, your excursions are limited. Your instructor will help you select a trip on which you can learn new elements of flying without getting frustrated.
Fortunately, you have a "road map," a sectional aeronautical chart for the area in which you live and fly. You can simply lay out the chart on a flat surface and draw a straight line between where you are and where you want to be. That's your course. You may modify it to take advantage of mountain passes or large navigation aids (lakes, rivers, mountain peaks, etc.) depending on where you fly.