There are many agencies in California to help you.
You first need to decide if you want to adopt a baby or older child. Probably it is easier for singles who want a baby to adopt from abroad, but it is also more expensive: usually between 12,000-20,000 by the time everything is paid for.
If you would like an older child or what is called special needs child ( a child who may be older, drug exposed, needing to be adopted with her siblings) there is almost no cost involved.
See also
Adoption Assistance site.
There is a great book called
A Handbook for Single Adoptive Parents which has all kinds of information on the kinds of adoption, costs, etc.