Eating at a restaurant can be a challenge when you're trying to avoid eating meat or other animal products. The best thing you can do is be prepared: be prepared by knowing where you're going, be prepared to be forthright about your dietary needs (both to your dinner companions and your waiter), and be prepared to have an open mind. I have found the following tips helpful for any time I know I'll be eating out:
• Check the menu online before you go - Almost all restaurants nowadays have their menus available for viewing on their websites. Cuisine alone doesn't always tell you how many vegetarian options a restaurant has. (For example, a steakhouse can be an ideal restaurant for a non-meat eater--albeit somewhat unethical--because of all the veggie sides offered. Combine a few of those, and that's a fulfilling and nutritionally complete meal.)
• Order off the menu - Depending on the type of restaurant, they may be able to do an off-the-menu vegetarian entree. Sometimes that means a fully prepared dish, sometimes it means a selection of side vegetable dishes. Either way, if that option is available, take advantage because it means you won't have to compromise your vegetarianism for a night out.
• Combine starters - Soup + salad, appetizer + salad, etc. can be a very filling combo meal, especially with the size of plates at most restaurants these days. Add a side dish and you'll maybe even have more variety than a single entree.
• Ask questions, speak up - Many soups at restaurants are made with non-vegetable broth. If this is something you want to avoid, inquire about it with your waiter. Same thing goes for dips, sauces, and anything else that would use a meat-based stock. If you want a dish, but it comes with meat (bacon in a salad, chicken in a pasta, etc.), don't be afraid to ask for it without meat and/or replace the meat with another vegetable. And if you're feeling really gutsy, you can even ask for the price of the now-meatless entree to be deducted (I've never done that, but on the rare occasion the waiter will actually offer).
• Be flexible, creative, open-minded - The lack of vegetarian choices on a restaurant menu can make eating out a limiting and restrictive experience, or an easy (sometimes having less options is a relief, especially with a huge menu) and even eye-opening (perhaps allowing you to try new things you wouldn't have as a meat eater) experience. Your attitude going in will determine which outcome you face.