The standard way to create a track is to add percussion instruments to the Beat+Bassline Editor and instruments for melody tracks into the Song Editor. Then create different percussion loops using the Add bassline button in the Beat+Bassline Editor for each rhythmic element you want - a main bass line, a hihat line, special rhythms and breaks, etc. These are listed in the Song Editor as they're created. To make them play, click on the start bar in the track's timeline to create a new block element (segment). You can also drag in Beat+Bassline blocks to make the pattern go across multiple bars. The same technique can be used for repetitive bass lines, if you create them in the Beat+Bassline Editor.
For melody tracks, click in the Song Editor on the bar where you want a new melody segment and then double-click that block element to open the Piano Roll Editor. Place your notes in the piano roll, by clicking on the notes wanted start "position" or tick, to create a new note, you can then drag the notes right-hand edge to be as long as you want. You can also drag notes up and down to change their pitch.
Alternatively, you can press the record button in the Piano Roll Editor and play notes in real time, either on the computer keyboard or a MIDI keyboard.
We recommend that you as next read and do the exercises in the section Your First Song with LMMS.