Using ComboBox/ListBox in Form

I am a student using VB.Net for first time. I am having trouble writing code
for a ComboBox and Button interaction. I am doing a simple form for
displaying hotel charges. ComboBox has five selections-one through five
nights. The button shows total charges in a textbox. Select "Two nights" in
comboBox, Click on Total Charge Button and the calculation should display in
the textbox. The more I research the subject of ComboBox in books and at the
online help sites, the further away I seem in my coding. Any reference
steering or guidance would be greatly appreciated as I have really bogged
down in this area!