SMS modems (and some mobile phones) connect to the COM port on the PC and support 'AT' commands. These are simple text strings that form a standard 'language' that modems support. To make your VB program work you need to do three things:
Use the mscomm control to enable VB to talk to your COM port
Write an 'engine' that generates AT commands, configures the modem, processes replies, handles errors etc
Write a GUI interface
It's possible to do but quite a lot of work to get right.
For testing purpose, you can first use Windows Hyperterminal to "talk" to your mobile device via COM or modem interface using a "data cable". Check out this tutorial on using Hypeterminal - http://www.visualgsm.com/tutorial06.htm
Also, will you only be sending your SMS in ascii? If you need to send in unicode, you would need to use PDU to encode your messages. Another way is to download some of the available free or open source SMS senders and connect via API, e.g. http://www.kannel.org, http://www.visualgsm.com.