If you're a die-hard foodie, then your whole Los Angeles trip may be based around some specific restaurants you have in mind, and for those, you probably want to make reservations before you leave home. Opentable.com is a good resource for researching restaurants and making reservations. For the rest of you, where you eat will be more a function of what attractions you're visiting and where you are staying.
In addition to OpenTable, Yelp.com, and its mobile app are my best friends when looking for someplace nearby that has been well-reviewed by regular people. As a bonus, the mobile app tells me what's nearby that is open right now and there are sometimes discounts for checking in.
If you're a traveler, you might also want to compare reviews on TripAdvisor. Travelers seem to be more appreciative of certain restaurants than locals, perhaps because they have different expectations.
Here are some links to other resources for researching the trendiest restaurants:
LA Weekly Restaurant Section
OC Weekly Restaurant Section
LA Times Food Section
If you're not from another major city, you might find that prices in Los Angeles are significantly higher than you're used to, especially at well-known restaurants. It is still possible to find places where you can have an entree and a drink for lunch for under $10 and dinner for under $20, but you can also easily spend $100 per person, or more, for dinner.
Drinks can also be extremely pricey, especially in clubs. If you're used to a $3 beer and a $6 martini, the $8 beer and $16 martini might come as a bit of a surprise. So if it's going to be an issue, be sure to check the menu or ask how much before you order.
What I have to Say About Los Angeles Restaurants
I don't review too many restaurants myself because I'm a picky eater (I confess all my eating quirks here), but here is the link for my Restaurant Reviews on Los Angeles Travel.
If you want to check out some really landmark eating establishments, I've compiled a list of the most Historic and Iconic Los Angeles Restaurants.
For special occasions, I've put together a list of the Most Romantic Los Angeles Restaurants with a View and the Most Romantic Restaurants in Orange County.
If you want to eat like a local, you might want to be on the lookout for some of the Top Gourmet Food Trucks tweeting their next location around Los Angeles.
If you want to take your Los Angeles dining experience to another level, you can sign up to take a cooking class from a local chef.
If you want to plan your trip around one of LA's signature foodie events, take a look at LA's Top Food and Drink Festivals, or plan to visit during DineLA and other Restaurant Weeks. If you're especially into ethnic food, check out Ethnic Festivals in Los Angeles.