Nearly all radial-arm saws are designed with a metal base, to which a replaceable wooden surface can be affixed. There are a number of reasons for this design, but probably predominant in the list is that for many cuts, the blade of the radial-arm saw will actually cut into the table top. Using a surface made from wood will protect the saw blade and can easily be replaced when it becomes too worn out.
Best Surface Materials
I've found that my favorite surface for the table top is MDF or medium-density fiberboard. It is inexpensive, pretty durable (as long as it doesn't get wet) and it doesn't splinter when the radial-arm saw's blade cuts through it like plywood would.
Attaching the Surface to the Saw
You'll likely notice that there are a number of holes in the metal base into which you can attach bolts. The heads for these bolts should be recessed (into countersunk holes) in the top of the MDF so the bolts do not interfere with the working surface.
When installing a new surface, the first thing to check is to see that the travel of the radial-arm saw is even with the surface. To test this travel, set the angle of the radial-arm saw to 90-degrees, then lower the radial-arm saw until the blade is barely touching the surface of the MDF. Pull the saw forward. The blade should neither dig into the surface or lift away as the saw moves the length of its travel.
If it raises or lowers as you pull it across the surface, you'll need to adjust the saw (or in lieu of proper adjustments, you can place shims under the MDF surface as it sits on the frame).
Installing a Fence
Once the surface is true to the travel of the saw, the only thing left is to install a fence.
The fence needs to be installed perfectly square to the saw when it is set to 90-degrees. I like to use a 2-1/2" wide piece of MDF on edge. I simply make a cut in the MDF surface where I want the fence to reside and attach it to the large forward piece with a few MDF screws. The remaining space behind the fence can be filled with more MDF, bolted to the frame below.