My computer has been acting up lately so I haven’t been able to use Blender for a while now. I’ve come back to it and while I still retain much of what I had learned, there are some basics that seem to have gotten away. Right now I’m trying to model an object by doing only half of it and using the mirror modifier. When I click “set smooth” I get that really bad seam on the axis of reflection and I can’t remember how to get rid of it.
How many modifiers you have in the stack?
If you used Subsurf Modifier too, make sure it is below the Mirror Modifier
If that doesn’t solve it - please post an image with the problem.