Nice design m8 but you haven't told us what software you're using, and the question you're asking covers rather a large topic depending on what you want to achieve from slicing.
I use PhotoShop CS (which includes ImageReady). If you use the same, go to 'Help / Photoshop Help'. In the new window, go to the left hand Nav Bar and select 'index' and the 'S' link. Then look at the 'Slice' entries for more info.

The most basic suggestion I can offer is to have you select the 'Slice Tool' and use it to enclose the areas you wish to slice. You will see them appear as slices. If possible try to get the horizontal and vertical lines to line up. This isn't compulsory in any way but may eliminate optential problems further down the line.