It is easy for landlords and tenants to end a month-to-month tenancy in California. (The situation is more complicated when it comes to breaking a fixed-term lease.)
In most situations (with the exception of some properties in communities with rent control), your landlord does not need to give you a reason (although acting on discriminatory or retaliatory motives is illegal). A landlord can simply give you a written notice to move, allowing you 30 days (60 days if you've lived in the rental a year or more) as required by California law and specifying the date on which your tenancy will end.
Your landlord may legally provide less notice in specific circumstances--for example, if you have not paid rent, if you have violated other terms of your rental agreement (for example, bringing in an unauthorized tenant), or if you have violated basic responsibilities imposed by law (such as by dealing drugs on the rental property).
Unless your rental agreement provides a shorter notice period, you must give your landlord 30 days' notice to end a month-to-month tenancy. Be sure to check your rental agreement which may require that you give notice on the first of the month or on another specific date.
In some situations, you may be able to move out with less (or no) notice—for example, if your landlord seriously violates the rental agreement or fails to fulfill legal responsibilities affecting your health or safety.
Check California state law (Cal. Civ. Code § 1946 & § 827a) for the exact rules and procedures for how landlords must prepare and serve termination notices and for any special rules regarding how tenants must provide notice. See the Laws and Legal Research section of Nolo for advice on finding and reading statutes and court decisions.
The state guide to tenants' rights (see California Department of Consumer Affairs) may also have useful information on how month-to-month tenancies end.
Also, the Nolo book California Tenants' Rights includes detailed discussions of rules regarding tenancy terminations and evictions in California.