Pictures are always good.

When you say head tube, do you mean handlebar stem? Does the bike have a dérailleur hanger on the drop out? Is it really a cassette, or a freewheel? Can you count the cogs, 6 or 7? Is there a brand stamped on them?

I could go on with questions, but you get the idea.

Need more information if you want help.

Oh, and pictures of what you want to change or have questions about.