Welcome to the FPGA club!
Inside the starter kit box, grab the purple book, "Introduction to Programmable Logic". Read the section about the Spartan 3, and the tutorial chapter about ISE WebPACK.
Are you already experienced with digital design and HDL? Verilog or VHDL is the best way to enter your designs into an FPGA. The XST User Guide has a mini description of both languages that can help you get started.
You will probably have many questions. Search for answers in your ISE documentation, the Xilinx web site, Google, and here at EDAboard. If you get stuck, you are welcome to ask questions here.