I was a vegetarian for about 10 years and I found the less you say about it, the better. People want to quiz you and then offer you ridiculous substitutes. (My former mother-in-law once presented me with a bowl of microwaved frozen plain mixed vegetables for lunch at the side of the pool, when she had made turkey sandwiches for everyone else. She is pretty daffy and I think her definition of ‘vegetarian’ was ‘eats only vast quantities of unsalted vegetables.’ I should have asked her if the concept of pimento cheese sandwich was a foreign one to her.)
My view may be colored a bit by these in-laws, however. My policy after a while became just to say ‘no thank you’ when offered meat. You’ll also find that people will want to serve you fish. (A confusion, I think, born of Catholics “giving up meat” and eating fish on Friday.)
At home, I was able to subsist nicely on beans and rice most of the time. I loved them and can eat them any day in all their variety. I ate quite a bit of blackeye peas since they’re my favorite. I also ate a lot of salads and vegetable cassaroles.
In regard to eating meat on special occasions or when it’s inconvenient to a hostess to say ‘no,’ I say go head and do it. If it doesn’t make you sick to do so, and if you’ll enjoy it somewhat, go ahead. You’re not doing this to prove anything to anyone, right? I, for example, never fully gave up pepperoni (though I did go for months not eating it) simply because I irrationally love it so much. 
Soups can also be your friend. I agree with Opal, I think mentioning that you can eat a lot of familiar foods already that don’t have meat in them – bean burritos are one example. Think about vegetable soup; just leave out the chunks of beef or whatever you usually put in. You’ll never miss it. Also (back to beans) I used to make a lot of split-pea soups, lentil soups or squash-based soups. Pizza is another easy vegetarian entree.
Breakfast is another source of inspiration. Make an omelette for dinner, using mushrooms and other vegetables for the filling, and have a can of blackeye peas on the side. (One of my favorite dinners.)
I also second, or third, getting a nice vegetarian cookbook. Vegetarian Times makes a good one, but the one I have is pretty gourmet. I picked up a little magazine-looking booklet in the checkout line when I first went vegetarian put out by Betty Crocker, I think, called Easy Meatless Meals. They are very easy and simple.
Good luck. I’d love to go back, but like DB my family is agin’ it, and I don’t have the energy to prepare their meal, and then mine.