Is this a prebuilt system such as a dell, hp? If so, give the model and we can help.
If this is a built system by you or a local computer store you should be able to identify the motherboard by its labeling directly on the motherboard. Some cheaper boards don't have clear markings but its worth the effort to look anyway. You will also need to identify if its a micro atx or standard atx motherboard. Measure its width and length and post with your info if you can't find any labeling on it. As graywolf mentioned, you can get the info from the top of the cpu to help us identify your cpu socket type. This will help greatly if you can't identify the mobo...