I can’t use my iPhone as modem.

Dear All,
I'm using Apple iPhone 3G. I travel a lot and I always use my iPhone while I'm traveling and I have a internet connection through my sim card. I need to always connect with Internet. This is one of my job requirement.
Thing is iPhone has a very much good display but this is not sufficient for me. The resolution of the iPhone sometimes is not suitable to view any mail and attachment. So, I want to know how can I able to connect my iPhone with a laptop as a modem.
Is there any solution? please help me.
Thanks Sikder
