There are two major ways to buy treasury bills:
Online through the Treasury: TreasuryDirect.gov is the Treasury Department's official online source for investing, available to all permanent U.S. residents. Start by creating an account and verifying your identity — you may need to submit your SSN or taxpayer ID number. Once logged in, you can choose which T-bill to buy and the amount, with a minimum investment of $100 applying to all maturity terms.
Brokers and banks: You can also buy T-bills through a brokerage or bank, either at auction or on the secondary market. The bank or brokerage acts as an intermediary between you as the purchaser and the Treasury, similar to when purchasing shares of stock through a brokerage account.
After purchasing, you'll receive the full face value of the T-bill at maturity. Your profit can be calculated as the value at maturity minus the initial price of investment.