Attaching your blend would be 100 time more useful that your screenshots.
By default the object is mirrored around its origin (large dot), is the origin in the correct place?, check it
Is your object rotated, if so clear rotation with Ctrl+A or mirror a different axis
If you’re following a tutorial make sure you actually follow what is shown, not just think you are doing what the instructor is doing.
See the gap? Go into Edit Mode and select the vertices at the end of your mesh closest to the gap. Then Shift-S, Snap Cursor to Selected. Go back to Object Mode, find the Object menu at the bottom of the 3D View and select Transform->Origin to 3D Cursor.
Your mirrored geometry should pull itself together.
Oh, and while you’re at it, you might as well do this, too after doing the above: