Renting a car is the best to get around in Los Angeles. LA's public transportation is not as developed as many major cities from around the world. Given that LA covers such a large geographic area, a car allows you to experience a lot more instead of spending wasted hours on bus/train connections.
For example, you can get to Hollywood rather easily via subway from Downtown. But there is not a direct public transit route from the Hollywood area to the Santa Monica Pier. This example shows the difficulty a visitor might have it they want to go Hollywood and the beach on the same day. It especially makes it difficult to see a lot of sites if you only have a couple of days in Los Angeles. Reputable car rental company links below:
You should also do some comparison shopping and search various sites at once by using sites like Carrentals.com and Priceline. Make sure you do not book your car too early with a non-refundable rate, since rates tend to be at their highest a few months in advance. Your best option is to book a rate with free cancellation and continue to check every 1-2 weeks in case a lower rate becomes available. Then you can cancel and remake your reservation. This process is very easy on sites like Carrentals.com.