frequency meter

hi every body
i have an assingment to built an frequency measurment meter with pic 18f242 microcontroller ,from range 10Hz to 100KHz i have built the hard ware , and i started the soft ware which i have to writ it by C languge
i straggled in the writing the code of calculating the frequency code , so please i need help , if any body their have an example or any idae about that please help me ,

i now the in put port and the timer put my problem in the code and i am using MPLAB