This is a difficult one. There are some schools -- but not too many -- that teach etiquette, including table manners. But they can be pricey. There are books that might help. But, I think the best way is to confide in someone you trust who has the impeccable table manners you admire, and ask them confidentially for a little help.
There's really not an awful lot to learn. Some of the simple rules:
* When you sit down at the table (whether in a home or a restaurant), the first thing you do is put the napkin on your lap--even before you pick up the menu. Unfold the napkin halfway, not completely, and lay it across your lap.
* If you leave the table during a meal and will return, put your napkin on your chair, unless you're in a private home, then you can place it on the table. In a restaurant, a napkin left on the table means you are done with the meal; in a home, the host or hostess knows you'll be returning from the table.
* Learn to hold your knife and fork properly! This is my biggest gripe.
* In the US, we cut a single piece of meat, then switch the knife and fork hands and eat it. It's not done that way in Europe, and it's probably OK in many other places too.
* If you're buttering bread or a roll, break off a bite-sized piece, butter it, and eat it. Don't butter the entire piece before eating it.
* Learn how to set a table properly so you'll know which glass is yours and which bread dish is yours.
Hope this helps! Good luck! I admire your desire to improve.
You might take a look at the information here:
http://www.cuisinenet.com/digest/custom/etiquette/manners_intro.shtml