I assume you mean tethering your phone via usb or blue-tooth to a laptop to use your 3G for internet access. If so then there are several options.
1st - June Fabrics PdaNet application can be used for all versions of the Eris OS and all leaks. You can use Http (none secure) access without buying the program, BUT if you want to use Https (secure websites) then you will have to pay for it. Just look for PdaNet in the Marketplace. That is the one that I would recommend. Also search for tethering under the marketplace and you can see the others. There are other ways to do it if you have root access on your phone too. Do some searches as all of these options have been covered in plenty of detail here.
Hope this helps,
Monkymate