VREF and COMP pins for a UC3845
how much voltage VREF should have

VREF for the UC3845 is 5V. It is set by the chip, not your circuit.

plus, i dont know COMP pin use for, can anyone explain me ?

Figures 20, 21, 22 and 29 of the ONSEMI datasheet show a couple of uses of the COMP pin.
Also see Figure 8 and page 6 of the Fairchild datasheet for an additional shutdown mechanism.
The TI application report AN-1286 Compensation for the LM3478 Boost Controller contains an in-depth discussion of the theory behind the compensation network shown in Figure 29.