c - Kp, Ki, Kd for software PID control

I am designing a software PID control. Logic is almost final but I wonder how do I decide the value of \$k_p\$, \$k_i\$ and \$k_d\$. Also, I need to determine the max and min value for the Pterm Item and Dterm. How do I do that?. Also, I am trying to implement the inverse control. Also, am I going in the right way in designing the software PID? Also, if integral time and \$k_i\$ both are same? The code I wrote so far is given below.

Calculate_Error();
    P_Term = Percnt_Error;

    P_Term = KP * P_Term;


    if(P_Term >= PMAX)

    {
        P_Term = PMAX;
    }
    else if(P_Term <= PMIN)
    {
        P_Term = PMIN;
    }

// Integral calculation


    if(Integraltime==1)                 // Take integration at every 1s
    {
        Integraltime = 0;
        Error_Accum = Error_Accum + Percnt_Error;   // Error Accumulation over time
        if(Error_Accum >= MaxAccumError)
        {
            Error_Accum = MaxAccumError;
        }

        if(Error_Accum <= -MinAccumError)

        {
            Error_Accum = -MinAccumError;
        }

        I_Term = (Error_Accum)*KI;

        if(I_Term >= IMAX)
        {
            I_Term = IMAX;
        }

        else if(I_Term <= IMIN)
        {
            I_Term = IMIN;
        }
    }