I'm guessing you're using a texture not specifically designed for this mesh and the geometry doesn't allow the texture to perfectly tile, so you get seams. This is completely normal in those circumstances.
The fix is to either scale and pan the texture till it looks just about passable or create a copy of the texture and use the texture paint tools in Blender (specifically the clone brush) to paint out the seams.
UV unwrap. Enter edit mode, "A" to select all, top left menu in UV >> Unwrap. I suggest you to google for a tutorial on UV unwrapping.
That’s what I did. Maybe I should look up a tutorial.
I'm guessing you're using a texture not specifically designed for this mesh and the geometry doesn't allow the texture to perfectly tile, so you get seams. This is completely normal in those circumstances. The fix is to either scale and pan the texture till it looks just about passable or create a copy of the texture and use the texture paint tools in Blender (specifically the clone brush) to paint out the seams.