If you are going to be doing woodworking on a regular basis then you may find a good woodworkers bench a valuable and important "tool" to have.
(Photo from Finewoodworking.com)
A solid heavy workbench provides a stable surface to work on and the ability to hold down or onto the piece you are working on.
There are different styles of bench's but most have Two common features that are for holding wood, A vise, or three, and bench dog holes for bench dogs and hooks.
These offer different methods of holding a work piece depending on its size or how you need to interact with it.
Until you can build or buy a good workbench you can come up with alternatives like some of the other answers here. A planing stop on any old work surface would work well, you can buy one like this one or just come up with your own idea.
You could also use bench cookies, they look like a hockey puck but have a nonslip coating that grips the bench and the work piece or accepts accessories for bench holes or clips.
I am a customer of Rockler and like there products but there are many good woodworking tool suppliers out there.
Also, you may find that the Woodworking stack exchange has deeper knowledge base for woodworking specific questions.
Here is good YouTube video on sizing a workbench that you may find interesting.