Depending on the location of your studies, the options for securing funding can vary widely. In general, however, funding takes one of three forms: a stipend, payment for teaching assistance or payment for research assistance. Moreover, depending on where you are studying, you might need funding to pay a large tuition fee, or else just to pay for living expenses. Some programs offer students a stipend with no obligatory teaching of research assistance in exchange. Such stipends can be need or merit based, or they might simply be included in an acceptance package.
More common, however, are stipends in exchange for work that is undertaken for the university. As a teaching assistant, one is generally asked to work with professors and to teach undergraduate courses in a relevant discipline. As a research assistant, you might work on an ongoing project in an area within your field. Working as either a research or teaching assistant offers valuable experience for your CV and future job prospects, making these positions rather competitive.
Many PhD candidates try to secure multiple forms of funding, either simultaneously or one after the other. When looking for funding, remember to look beyond your home institution to places like foundations, government sponsorships or international academic organizations.