Perform a general countywide search. One of the best places to look first is the Orange County official website, which has listing of all the county's local parks. However, the website will direct you to a map with a tree-shaped symbols that represent each park. However, the map is a little overwhelming, and it is tough to go through it all to find your perfect park. It is a great place to start, and gives you a bit of direction if you're not familiar with the different regions of the county (see Resources below).
Get a general feel for a specific area. Pick out a few parks that are close to you geographically and then learn a little bit more about them. For this I recommend a non-county resource which gives a general overview of the park you are interested in (see Resources below).
Narrow down your region, and look at some third party sites. A final amenities search is a good way to pick a specific park. Does the park have the right amenities? Is it pet-friendly? How much is parking? Since there are so many parks in Orange County, it is good to find enough specific information on the park you wish to visit. Many of the larger parks have their own websites (see Resources below).