Federal income tax applies to the profit, and how it is paid depends on the entity. A sole proprietorship or a single member LLC reports it on the owner's personal return. A partnership or an S corporation passes it through to owners. A C corporation pays at the entity level. Self employment tax, covering the social security and medicare contributions an employer would otherwise split with you, is the one that surprises people most, because it applies on top of income tax and is a meaningful percentage.
Because nobody is withholding on your behalf, you pay estimated tax quarterly rather than annually. Missing those deadlines produces penalties even when the eventual return is correct and the full amount is paid. This is the single most common tax failure among first year businesses, and it happens because the money was spent before anybody worked out that a portion of it was never theirs.
Nevada has no state income tax, which is a genuine advantage and not the whole picture. There is a state business licence to maintain, local licensing depending on your city and county, and a commerce tax that applies above a substantial revenue threshold most first year businesses will not reach. Sales tax applies if you sell taxable goods, and that money is collected on behalf of the state rather than earned, which makes spending it a serious problem rather than a cash flow decision.
Employees add payroll tax, withholding, unemployment insurance, and filing obligations with deadlines, all of which begin with the first employee rather than at some later size. A payroll provider handles the mechanics and the registrations, and at this scale using one is cheaper than the time and the risk of doing it manually.
Track it by separating the money rather than by intending to. Move a percentage of every payment received into a separate account the day it arrives, treat that account as untouchable, and pay from it quarterly. The percentage will be wrong initially and your accountant can refine it. What matters is that the money is elsewhere, because the failure mode is not miscalculation, it is spending funds that were always owed to somebody else.