There is no ADC inside the FPGA, you can integrate it with the FPGA and then transfer data from ADC to FPGA. (I haven't done this yet so I cannot assist more on this
any ways good luck.
To communicate you use ports, they are usually type
bit or
std_logic if you are using VHDL and if using verilog then
reg or
wire. FPGA programming is not conventioanl computer programming so you will be dealing with binary digits, so if you have decimal 12,
you will be looking at it as binary 1100 etc.