FPGA design is digital logic design. That's very different thinking than programming a microprocessor in C. You need to understand counters, multiplexers, sequencers, state machines, accumulators, synchronous and combinatorial logic, timing issues, and other stuff like that. If you don't yet know digital logic design, you should learn it before tackling an FPGA project.
There are some software tools that try to convert C programs into FPGA logic, but the translation has many limitations. You can get much better results (faster performance and smaller size) by designing your FPGA logic using an HDL such as Verilog or VHDL. If you enjoy C programming, you will probably find Verilog easier to learn than VHDL.
Another approach, you can buy an inexpensive development kit from Xilinx or Al.tera (approximately $150 US), study the tutorial examples, and run them on the FPGA board. That should give you a taste of what you need to learn.