Check out this playlist of 30 videos by Bob Purcell (Chief Threadologist of Superior Threads). He’s a very engaging speaker and knows more than you ever would have imagined about needles and threads. Excellent! Excellent! Excellent!
Superior Threads is one of my very favorite sites. I love their threads, of course, but they are such a wealth of information. They also have 3-month classes. Check out their page of Troubleshooting Tips. There are tips about Everything related to sewing machines, and they include sergers in their tips as well. It is a great go-to resource.
Even more tips comes from their Reference Guides, like how many quilt blocks are needed for a particular quilt size, how much thread is needed to quilt the project, etc.