You have to do it differently depending on the version of VB.
in VB 6.0 (before .NET) you use the seriel comm. component this is the easiest way.
in VB 2005 (the newest .NET version) you use the system.io.ports.serialport class
in VB 2003 (the "old" .NET version) you are in trouble. The system.io.ports.serialport class is not there, so the only way is to use the VB 6.0 component but this requires some fairly hairy marshalling and what know i.