Glass is available in a variety of thicknesses. The thickness you choose depends on things such as the amount of exposure to wind (and the suction load on the surface) and the size of the sheet required. Most domestic glass is 4mm thick.
In most cases, any glass you need will be cut by the glass merchant, with you just supplying the measurements. However, there may be times when you need to cut class yourself and it is important you know how to cut it correctly.