1) Find a location
I like to use a combination search and verify strategy to determine, which restaurant would be perfect for the night or event I am planning. I first create my itinerary for the day and then try to figure out where exactly I will be around meal time. Will I be out and about near a specific museum, just coming into a city, or relaxing at my hotel? These all factor into where I may be looking to eat. Will I be celebrating an important event or just looking for a quick bite? If I'm celebrating, then the location might not be as important as the experience or quality of the food (sometimes it might be even more important, i.e., Eiffel Tower). These are all considerations you will have to make before you begin locating dining options.
2) Look for the website or online materials

3) Reserve online

Another option is to use online tools to make or request a reservation. I have used TopTable, Book A Table, and The Fork (for France), for my own reservations. In some cases, these tools even provide a coupon or discount for the establishment, saving you some money. Keep in mind that a little bit of research can go a long way. Before going directly to the online tools and making a reservation, read the full reviews available on TripAdvisor, Yelp, Zagat, Urban Spoon, or Dine.