If you have been certified to give first aid, you need to retain the certificate and update it with the update training every two years (easy; if I can do it, an ape can).
If the patient is conscious, you need to get verbal permission to apply first aid, always. Preferably in the hearing of a witness. Then you are covered by the Good Samaritan Law, which is fairly strong and widely acknowledged in CA. If the patient is unconscious and unable to give verbal consent, you are covered by the "implied consent" clauses of that law.
I'm NOT a lawyer, but am simply repeating the instructions of our first aid trainer, who makes his living as an EMT and has for many years, and of course is a certified FA/CPR instructor, a guy much more likely to get sued than you or me.
And you can ALWAYS be sued in our society, for just about anything. So when giving first aid, you need to act like a doctor--take your training seriously, and do your best to
"FIRST, DO NO HARM." So if you literally "don't know what you're doing," don't do it--call 911 and get a pro, then try to make the patient physically stable and comfortable as you can.
If you're trained, though, you have a moral obligation to try to render aid. Just my opinion, but based on a couple of thousand years of ethics.
It is also my opinion that everyone who is physically capable of rendering first aid should get trained and also equipped with a decent kit. And carry it with them at all times.
Why? 1) The training and the many issues it raises are 3000% more interesting and real than any "reality TV." 2) Sooner or later, we are going to have one hell of a disaster in CA. 3) In the meantime, real people get hurt every minute of every day, and they deserve help.