Try these at Park Tool. Sometimes they ham it up to much, but Calvin seems to always give good information.
Front Derailleur Adjustment
How A Front Derailleur Works
Front Derailleur — Advanced Troubleshooting
You can find all sorts of decent information and videos at the Park Tool website.
https://www.parktool.com/en-us/blog/repair-help
A little bit more information about your bike, it's components and what the issue is might let some of us try to guess what might help you or fix the issue.