If you are using EMGU, as it says in your tag, you can call cvSmooth. I have discussed the parameters below and shown an example of the call.
Image<Gray, float> inputImage;
Image<Gray, float> smoothedImage = new Image<Gray, float>(inputImage.Width, inputImage.Height);
int apertureWidth = 10;
int apertureHeight = 10;
cvSmooth(inputImage.Ptr, smoothedImage.Ptr, SMOOTH_TYPE.CV_GAUSSIAN, apertureWidth , apertureHeight , 0, 0);
for an explanation of the call to cvSmooth.
You will need to select the smooth type as explained here
Also read this
] on openCV Image Filtering, cvSmooth is about 3/4 of the way down the page.