At the center of the Studio, you find the arrangement with the timeline, loop indicators, and playhead. The arrangement is the area where you drop in library regions to tracks, record your own audio or note content and create new empty regions. You can quickly create a new region on a track by double-clicking any empty space in the arrangement on that track. You can also create a new track and a new empty region when double-clicking the empty space below the last track in the arrangement.
To use content from the sound library simply click and drag a file to your arrangement. Right above the arrangement, you have the timeline looking a bit like a ruler with measures in musical bars. You can click the playhead to move around or use the yellow loop indicators to repeat or loop a certain section of your song.