Hi,
It's actually quite simple to setup taxes for California in HikaShop. Here is a step by step:
1. Go in System->Taxes, click on "manage tax category"
2. If you don't have a tax category, you need to create it (you should already have a default one).
3. Go in System->Taxes, click on "manage tax rate"
4. If you don't have a tax rate, you need to create it.
5. Go in System->Taxes, click on new in order to create a new tax rule
6. Select the zone of the tax (California), the rate, the tax category, publish it and save.
7. In your products select the tax category (the default one should already be selected).
8. Set a "main tax zone" other than California in the configuration page of HikaShop (it should already be the case actually).
You tax is now setup.