You haven't said anything about what your system consists of or what problem you are referring to.
If you are referring to the IR starting before t=0, that is because the timing reference is closer to the mic than the system you are measuring. You can compensate for that by adjusting the timing offset on the Measure dialog, the Estimate IR Delay feature can calculate a value for you to use.
If you are referring to the oscillatory nature of the IR and your output and input are on different devices that may be due to slightly different clock rates on the devices, which causes dispersion in the IR. If you measure with an timing reference REW can correct the clock rate difference. Select "Adjust clock with acoustic ref" or "Adjust clock with loopback" in the
Analysis preferences depending on what you are using as a timing reference.