Combobox selection populating usercontrol

Here's my situation:

I have a main form, frmMain, which has a tab control placed on it. I have made user controls that are then placed on each tab.

Above the tab control, on frmMain I have a combobox & textbox.
The user will select from the combobox and the text box will be filled with the description of the item selected in the combo.

I need to have the cooresponding data to the item selected in the combo box filled into the usercontrols on each tab.

The code for populating the combobox is in the frmMain load event. The code to populate the user control is on the usercontrol. The question is how do I get the value in the combobox passed to the usercontrol so it gets filled correctly?

I'm still not sure how to pass data between forms. Should the code to populate the user controls be placed in a class?

Thanks