Hi, it's amazing that you saved up quite a bit of money and started to look for properties. I aspire to do the same once I graduate the university I'm attending.
Since you're in an expensive area(California) and looking to invest with 15-25k, it would be pretty hard to find properties, especially on the MLS. You might be able to find properties that are a little distanced to major cities in California.
I read from Brandon Turner's 'The book on renal property investing' that even in expensive markets, people who earn relatively low income (for ex. Baristas, mailman, etc) still need a place to live. I would try to identify those markets in California. Then again, since California is a difficult place to invest for beginning investors, you would have to do a lot more due diligence. The laws are not landlord friendly. I know there are some investors who are thriving in the California market, and if you could get to meet some of them, that would help you greatly.
I think California is a great market for those who can identify opportunities that most would shy away from, or cannot identify. Although California is loosing a great number of people, the state is still a huge economy, and those who identify hidden opportunities and have experience will thrive.
I agree that owning a property and not being able to find 'good' tenants is scary. Hence the reason why it's a good idea to buy a house hacking property where the monthly payments on an FHA loan over 30 years would allow you to 'survive' if you made mistakes and could not find tenants, or overpaid for a property.
As for the questions, I'm not a investor yet, but I did read everything I could get my hands on about real estate, and I believe:
1. It's recommended to buy your first property through the MLS even if that means you're not getting a GREAT deal because you are likely to make mistakes on buying your first property, and you would rather buy with an agent's assistance than on your own.
2. Advertising yourself, such as on websites like Zillow, or Apartments.com, or just hiring a reputable property manager to advertise for you. I heard putting 'For Rent' signs on the property might get your property vandalized(because it means it's empty), so it might not be a great idea.
3. When I analyze deals on the market, I use the BiggerPockets Rental Calculator. I would watch a few videos on youtube from BiggerPockets on how they use the calculator, and start analyzing 2 deals a day, maybe even more. When you start using a calculator, you would know how to estimate expenses like Capex, utilities(multifamily: if the utilities are separated), repair costs, vacancy, etc.
Lastly, it's really important to have cash reserves of 6 months of expenses before you get started.
I hope this helped, and please correct me if I'm wrong! Thanks :)