Extension tubes do two things you may not yet have noticed.

1. They reduce the amount of light reaching the sensor*.
   This is often enough to prevent autofocus from working at all. It's usual to abandon autofocus altogether for extreme macro & do everything manually.
   *This is because you have a much smaller area to capture light from, not because the tubes physically restrict light in any way.

2. They bring the farthest focus point much closer. This depends on how much extension you have & how long your lens is initially. Longer lenses don't suffer as much as normal to short lenses for this, but infinity is lost with any extension.
   With a standard set of extension tubes, 12mm, 20mm, 36mm, then on a 50mm lens, if you put all 68mm of tubes on - your farthest focus is actually so close it's inside the lens!

To get used to how this all works, start with one of the shorter tubes. This won't give you as much magnification, but as you gradually step up the tube section lengths, you will start to get a feel for your maximum focus length.
You'll also start to notice that the focus ring itself doesn't really seem to do much any more.

With 36mm of tube on a 50mm lens, your focus will be at about 2½", 60mm. Your minimum to maximum focus distance using the ring is probably less than 3mm. You've reached the point where it's almost become pointless. As you breathe things will go in & out of focus.

This is where macro starts to become as much art as science. You either practise at framing rapidly if your subjects are likely to get spooked & fly away, or you do still life and buy a tripod ;)
If you're trying to capture insects that will fly away, you would actually fare better with a longer lens, as you don't need to get so close.