The public transit system in Los Angeles is poorly developed and not the safest because locals do not use it regularly (compared to more dense cities like New York, Hong Kong or Singapore), and traffic can be terrible. As a result, it's incredibly important to plan your itineraries and accommodation in advance based upon traffic flow.
TIP: based on your itinerary, choose your accommodation near your preferred destinations! More than in other cities, it is important to be close to the areas you want to spend the most time, otherwise you will spend time/money on transit.
TIP: plan your itinerary contrary to traffic, especially during rush-hour. Traveling from East to West is much easier during the mornings (for example, from Hollywood to Santa Monica) than during the afternoons, and during weekdays than weekends.
TIP: depending on your length of stay, rent a car. The cost of Uber / Lyft can add up and become more expensive than renting a car if you are staying for more than a few days and have a packed itinerary. Having a car is also great to see more than just sites in downtown Los Angeles.