Some general thoughts...
Digital files can do strange things at edit points when the bits don't match up. Sometimes, slightly moving the edit points will eliminate digital pops. Having clips that are poor quality, single-channel source files with hum and very low volume can create issues.
It's best to record properly of course, with equal volume on left and right channels. But when audio post production is needed, it's wise to normalize volume, minimize noise and equalize prior importing for edit. Wavepad can help with audio pre-production. An improved audio file can be added to a new track and synchronized. When the original is muted, only the new audio clip will be heard.
During the edit, if you make a mistake while editing, immediately press <ctrl-Z> (EDIT|UNDO) to restore, step-by-step.
Go easy on grouping; it can cause unexpected behavior when clips are both grouped and linked. Use as few tracks and effects as practicable and do not load files that won't be used in the project; that just slows things down.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite.
Good work! Takes practice ... and patience.