I've bought a blackberry bold 9700, and I have 3G service. how can I use the internet with my computer?
Many phones support what's called USB tethering which essentially means that the phone shares its Internet connection with whatever USB host it is attached to. I'm not familiar with Blackberry devices, but USB tethering is a keyword you can search for in the manual and in the phone's settings.
For reference, USB tethering was deliberately left out of the iPhone for several years (although it could be done by "jailbreak" it), but eventually Apple realized that it's a smart feature to offer. Android also offers USB tethering, so I'm sure that Blackberry can do it too.
Look in your phone's network settings for something you can turn on.
After that, the computer should be able to detect the shared Internet and just use it. Look in System Settings > Network.