Actually the situation with LLCs in the US can be more complex. How you are treated by the IRS will be a function of the elections you choose to make, and you will need to file forms to make those elections. To quote from Wikipedia:
An LLC with either single or multiple members may elect to be taxed as a corporation through the filing of IRS Form 8832. After electing corporate tax status, an LLC may further elect to be treated as a regular C corporation (taxation of the entity's income prior to any dividends or distributions to the members and then taxation of the dividends or distributions once received as income by the members) or as an S corporation (entity level income and loss passes through to the members). Some commentators have recommended an LLC taxed as an S-corporation as the best possible small business structure. It combines the simplicity and flexibility of an LLC with the tax benefits of an S-corporation (self-employment tax savings).Source: Wikipedia - Income taxation Further depending on what you sell and where you sell it, you may be subject to state and local sales taxes which are independent of income taxes.
In general, a company will pay tax on profits, then the residual funds will distributed to the shareholders as dividends, who will then pay personal tax on their income.
Of course, the specific rules vary widely depending upon the jurisdiction and the individual circumstances.
My answer is based on the following assumptions:
Steve is right, you will pay income tax on profit, not revenue. However, his answer is inaccurate, assuming you stay as a pass-through entity.
First, there is no such thing as shareholders and dividends in an LLC. Those are terms used for corporations. Secondly, and more importantly, in your case you and the company are one in the same. As far as the IRS is concerned, your single member LLC is basically treated like a sole-proprietorship. This means that the LLC's profits will flow through to you as the owner, and you will pay your personal tax rate on the entire amount. With pass-through entities you pay taxes on your earnings only once, unlike with C Corporations, which is the process Steve described in his answer.
Another important point to make is that with pass-through LLCs you will have to pay income tax on your entire profit, regardless whether you choose to withdraw it as a distribution for yourself or keep it in the LLC for growth.
These other slightly related questions may also be of interest to you: