SIM cards
SIM cards can be purchased in any shopping malls. The closest mall is Smith Haven which you can get to by bus. This mall has outlets for Verizon, TMobile and AT&T which are the major service providers in the US. All of these networks have decent connectivity in this area. When you go to buy a SIM, they usually ask for an ID (international ID is accepted), a deposit (around $50), cost of a new SIM card (~$25) and SSN number if you need to buy a phone on contract with the network.
Most networks in the US come with unlimited text and call to anywhere in the US, Mexico & Canada. The data plan is what decides the monthly cost. For example you can opt for 1GB as we are usually connected everywhere by WiFI. Also, when you have a 1GB plan and it gets exhausted, you still have internet connection but at a lower speed (2G data).
An individual data plan costs around $40-45. It is cheaper to combine with other students or family members in the US.