Several years ago I was dining out with friends at a local restaurant, and upon finishing his meal, one of my companions proceeded to stretch his arms over his head, arch his back, and yawn, then slump back down in his seat. I was stunned, and embarrassed to be in the company of such a rude person who obviously lacked good social manners.
When having a formal dinner at a person's home, never sit at the dining table until the host is seated or you are asked to be seated.
Watch your hostess. You may begin eating when she begins eating, and you may leave the table after the hostess stands to leave the table. A good way to remember this is "Ladies First".
There are many good tutorials online that will teach you about formal place settings and which glass, or fork to use. However, when in doubt either watch your host/hostess, or start with the fork/spoon on the outside, and work your way in.
You should never take bites so big that it requires you to stretch your mouth open (certain foods like hamburgers, or stacked sandwiches are an exception). Always cut your food into small pieces, eating one piece at a time. This will enable you to chew quietly and quickly, and help you to maintain good table conversations. Chewing loudly, with your mouth open, or belching is UNACCEPTABLE, and will even cause the other guests feelings of disgust. I know that occasionally your body may work against you, so a simple "excuse me" is appropriate, and you may then continue the conversation with ease.
As I mentioned before, sitting up straight while dining has many benefits. You should always bring the food up to your mouth, not your mouth to your food.
Having good table manners is more then a bunch of rigid rules that some old people invented to keep up pretenses. It is a means to help you dine comfortably and still look good to the other guests. The primary goal of table manners is to ensure that every guest has an enjoyable experience. The good news is, even if you were never taught good table manners, it's never too late to learn!