Most new Sprint phones are designed with a SIM card slot. There are three different SIM card sizes however, only one will fit in your SIM card slot - you'll be able to choose the right one for your phone model easily enough when you buy.
There are a couple different ways to purchase your SIM card from Sprint. One way you can purchase your SIM card is to head into a retail store to and purchase your SIM card directly. The other alternative is to purchase is to visit the Sprint website. On their website, you'll see the the SIM card available for purchase.
If you purchase the SIM card directly from the Sprint store, you can simply pop the SIM card in your unlocked Sprint phone. If you purchased the SIM card online, your SIM should arrive in the mail within the 3-5 business days. Theres a simple activation procedure that you can do online to get your phone and new number working
There are also a lot of carriers that operate on the Sprint network- these carriers often have great priced plans but are generally cheaper than Sprint plans - you get the same network, the same speed etc you are just saving some money or getting more value.
Below is a link to see all available carriers and plans that will work with a Sprint phone: