Email alerts for approaching and past due dates

Email alerts for approaching and past due dates

I am working on a way to track required documents for a small fleet of fairly large boats. I need to have emails sent to alert us as documents are coming due. Along with this I am using a Action Tracker to alert to approaching dates repairs are due. If anyone can help me make it clear and simple I would appreciate it. I am not a computer guy I am a boat Captain so simple is better.