Gold was discovered in California in 1848, leading to the gold rush of 1849 (that’s why they’re called the Forty-Niners, people). California petitioned for statehood in 1849, and was admitted in 1850.
Prior to 1848, California was a territorial possession of no real worth other than its natural harbors (San Francisco and San Diego), acquired from Mexico as part of the settlement of the Mexican-American War in 1846. The population at the time of the cession was perhaps 10,000 people (not counting Native Americans, of which there were probably between 25,000 and 100,000), mostly hardscrabble cattle ranchers. Most local governments still conducted business in Spanish.
The main reason the US wanted California, prior to the discovery of gold in 1848, was to keep it out of the hands of Mexico and of Russia, and as a fulfillment of the Monrovian doctrine of “manifest destiny”: one country spanning the whole continent, sea to sea. Aside from the abundance of natural resources, fulfilling manifest destiny was essential for America’s long term security. Economically, ports on the Pacific Ocean provided essential access to Asian markets that would otherwise have been expensive and vulnerable. Geopolitically, incorporating the coast cut their land borders to just north and south, which is a big deal. Today the idea of the US at war with its immediate neighbors is borderline ridiculous. In the mid-1800’s, however, we had just seized the whole southwest from Mexico in a war, and had already fought Canada twice (and failed in invading both times.). Given the history of Europe, it seemed not only possible but inevitable that local rivalry would again boil over into war with our neighbors (an underrated reason the CSA absolutely could not be allowed to exist; war with a new rival for western expansion seemed impossible to avoid, so the civil war was in part a question of now instead of later.)