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 three largest 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 if you need to buy a phone on contract with the network.
Most cell service plans in the US come with unlimited text and calling to anywhere in the US, Mexico & Canada. The data plan is what decides the monthly cost. For example you can probably opt for 1GB as we usually have WiFi everywhere on campus and at home. Also, when you have a 1GB plan and it gets exhausted, you may still have internet connection but at a lower speed (2G data).
An individual data plan costs around $40-45. It is cheaper to get a family plan with other students or family members in the US.