your question isn't clear enough so I'm just going to take a guess.
to use a combobox, drag and drop it on the form in designer view. Once done, in code you can add items (or from the designer view, in the combobox properties...which ever you like):
Me.theComboBox.Items.Add("item")
of course replace "theComboBox" with the name of your combobox control.
You can also remove items:
Me.theComboBox.Items.Remove("item")
You can also implement different events (such as selectedindexchanged) to do particular things when a combobox raises an event, to which you may have implemented.
You can also get the item chosen:
Me.theComboBox.SelectedItem
or to get the text chosen:
Me.theComboBox.SelectedText
please post back, stating clearly what you would like to do with a combobox to better provide you an answer :-)
Combobox Class
http://msdn2.microsoft.com/en-us/library/system.windows.forms.combobox.aspx
Contains an Items Collections
http://msdn2.microsoft.com/en-us/library/system.windows.forms.combobox.items.aspx
With can have items added / removed and cleared.
http://msdn2.microsoft.com/en-us/library/system.windows.forms.combobox.objectcollection.aspx
These include add , remove, clear methods and many others which can be easily addressed. The items collections for the combobox works in a similar manner to those of many controls which include items collections.
An example using the combobox control
http://www.startvbdotnet.com/controls/combo.aspx
Best advice is to try it out and play with the control - provide more description about a specific problem which we can try and help you with rather than a very open ended how do I use the combobox question.