@Louise1073 @Emilie Yes, I live in San Diego too. Airbnb collects the transient tax from the guest and passes it on to the relevant city department, so you do not need to collect from the guest yourself.
For paying your income tax Airbnb provides a statement for this purpose on your earnings page so you can see how much income you had.
For expenses you can do a simple profit and loss calculation and submit it with your tax paperwork. I've only needed to do this when buying bedding, towels, etc. when I started, or when replacing these items. I don't bother for small items such as shampoo, flowers, and such.