Symbian

 Depending of your mobile phone's operating system you can achieve this by using tethering applications. On rooted android phones as well as windows mobile devices you can use 'Wireless Tether for Root Users' respectively 'WMWifiRouter'. If you call a jailbreaked iPhone your own you are lucky too. Some tethering applications do exist for Symbian phones either.  In all these applications you should enable the strongest possible encryption mode to ensure you are the one and only using your data plan. It is also recommended to enable a notification when any device is trying to connect to your phone over WiFi. Of course you have to create an according WiFi connection and add encryption keys on your notebook.  To not exceed your data plan's limits you should use a traffic counter on the cell phone. Also it should be noted that using a 2g/3g connection and a WiFi connection at the same time the battery of your mobile device will drain fast so prepare to surf fast.