I've been thru this stuff a few times so hopefully I can help...
In general, it is good to have a corporate entity (of any type, C-Corp, S-Corp, LLC) to legally seperate you from your business. If a customer, vendor or employee wants to sue the business for whatever reason, your personal & family assets are generally fully protected (unless you committed some type of illegal activity or fraud).
As for the different corporate entities.
A C-corp is geared for big companies, often times with shareholders. A C-corp is its own tax entity - that is, it must file and pay its own taxes. There is usually a "double-tax" effect with a C-Corp. Say a C-corp makes $1m in profit, it will pay tax of around $400,000 on that profit. The corporation then pays the balance of the profit ($600,000) to the owner or shareholders as a dividend, on which the individuals pay personal income tax of another $240,000. So $360,000 of the $1m in profit actually reaches the owners. Also, non-human entities can own shares in a C-Corp, such as trust, another corporation, etc.
Chances are a C-Corp is not for you.
An s-corp is not a separate tax entity, the profit from the business is filed and paid for by the owners of the s-corp. So say an s-corp makes $1m in profit, that $1m will show up on your personal income tax (via a schedule K1) on which you would pay $400,000 and pocket $600,000. No double tax. The trade-off, however, is that the tax liability is now on you personally. Say the company makes $1m in profit (on paper) but only has $200k in cash and and $800k in accounts receivable (customers own you money). You'll have the $400k to pay the IRS but will not have enough cash to do so. Also, say you decide to shut-down the company. With a c-corp, anything owned to the IRS dies when the company files for dissolution. With an s-corp, the tax liability does not die when the company is shut down - it will stay with you. Also, s-corp owners must be human beings - no trusts, no other corporation, etc. as shareholders.
BTW, an s-corp has a one-time election to convert to a c-corp. A c-corp cannot convert to an s-corp.
What about an LLC? An LLC with a single owner (called a member, not a shareholder) is what the IRS calls a disregarded entity, that is, you can handle the filing and paying of the LLC's taxes on your personal tax, same as a DBA. With multiple members, the process becomes similar to the s-corp. Unlike an s-corp, an LLC may have members of the human and non-human ilk. There are other benefits of an LLC in terms of managing decision rights and profit sharing rights amongst the owners (like law firm with various levels of partners and partner bonuses), but those probably don't apply to you, unless you are going to have "partners" in your business.
In california, all corporations must pay an annual state franchise tax. The LLC franchise tax is either $800 or $1300 per year. S-corp and c-corp tax is based on the number of shares the corporation has created, but figure a similar amount annually. If you are thinking - "I won't be a California LLC, I'll be a Nevada LLC" - if the company is based in California you will need to register as a "foreign" LLC operating in California and will be subject to the same franchise tax.
In summary:
C-corp; Another complete set of taxes to file annually (state and federal). Double tax on profits. Company retains benefit of losses. Annual franchise tax. Full protection of pesonal liabilities and assets. Cannot "write-off" home employment and self employment costs on tax. About $1k in franchise tax.
S-corp; File taxes with your personal tax. No double tax on profits. Also recognize losses of the company on your personal tax (assuming that you have invested at least as much as the loss). Company tax liabilities become your personal tax liabilities. Convertable to c-corp. Cannot "write-off" home employment and self employment costs on tax. About $1k in franchise tax.
LLC; File taxes (if you are the sole owner of the LLC) as if you were a sole proprietor or DBA. No double tax on profits. Can "write-off" home employment and self employment costs on tax (as long as you are the sole owner). About $1k in franchise tax.
You can always start up as a DBA. Until you have customers and/or employees, there is really no one to come after you with a potential lawsuit, nor any tax liabilities. When the business grows, form either a S-Corp or LLC. An sole-owner LLC, from a tax standpoint, is the same as a sole proprietor. As the company grows, the LLC can evolve to meet your needs - partners, investors, etc. No franchise tax (double check that in your county, it may be different).
Hope this helps.
