Welding Other Half Of Car After Subsurf

I have made a car and now have used subsurf and the middle section, where the line of symmetry area is, the two halves seperate from each other, I’ve heard you can weld those sorts of areas, HOW IS THIS DONE?

Zoom Waaaaaaaaaaay In, and move the two halves just together.
Then ensure the two halves are the same object.
In edit mode, select all vertices, and Hit Mesh>Vertices>Remove doubles. If it says something like “13 Removed” it probably worked. To check, RMB on the welded vertices, and if you can only get one vertice to select in one place, it probably worked.

Alternately, you could select all the vertices that would make up the middle line of the car and scale them to 0 on the axis of your choice (depending on what alignment your car is on with the axes).

You can also select the relevant edges and bridge them with F, making new faces to fill in the gap.
If you did it right, you can edge-loop select a lot of them at once.

Similar to the first reply you can also Weld the vertices, which is…er… er… brain freeze. Do Search.