UserControl is a scrollable container that is usually used for putting  
other controls inside it.  Unless you are specifically needing that  
behavior for some reason, you should inherit Control, rather than  
UserControl (I know, the name's confusing...but UserControl isn't a  
general-purpose base class for custom controls).

As far as implementing a text box goes, it depends on what you want to  
do.  How much of the functionality that TextBox has do you want to  
create?  How much work are you willing to put into it?  Even for an  
experienced programmer, this is a good week-long project just to get all  
of the various features implemented, never mind tested and debugged.  If  
you just want a subset, it could be a lot easier, depending on what you  
actually want to do.

As for how to go about it, you'll need to override things like  
OnKeyDown(), OnPaint(), etc. to handle key input and drawing the control.  
Short of actually writing the control for you and showing you code, I  
don't know how else to answer the question.  If you're unfamiliar with  
writing custom controls, I'll suggest that implementing a text box may not  
be a good first effort.  You might want to write a simpler custom control  
first, learning each part as you go.

Pete