Where do we need to apply for probate if we need to ?

Generally, you will need to open probate in the County in which your father was living at the time of his death. You indicate that was outside California - contact a Probate attorney in that County for more specific information about that state's probate rules. Whether or not a probate is necessary will depend on the probate code of that state.

You don't mention where the land was - if it was in that state, as well, then the transfer of the property will be run through that probate. If the land was here in California, or in another state outside of the one in which your father was living, you will need to open a *second* probate, usually termed an ancillary probate, in the county in which the land was located.

You're not required by law to hire an attorney to assist you. You should at least have the will reviewed by an attorney to ensure it's valid and to discuss whether probate is required. And if you do hire an attorney to assist with probate, at least the expense of the attorney will be paid by the estate, and thus shared between all the beneficiaries.

Probate is not always a bad thing - it can be very effective in heading off any creditors your father may have had, for example.