Most PhD programs come with full funding which includes paying your tuition, providing benefits such as health insurance, and giving you a small stipend. In return, you will typically be asked to be a Teaching Assistant or —and these are highly prized—a Research Assistant for a professor.
Even though you are likely to be fully funded, consider applying for NSF’s Graduate Research Fellowship.
This application is due well before the graduate school application cycle. NSF grants are extremely competitive but the benefits are large if selected. First, having an NSF grant is a great signal to admissions committees. Second, the funding is often more generous than you would otherwise obtain. Finally, an NSF grant will reduce your teaching requirements during graduate school
If you are accepted to multiple PhD programs, you should consider asking for some additional resources from your top choice. Please talk to your letter writers about this option first.