The first plan selection.

[micheus]

Ok. I got it.

It seems you used the marque option for the selection - by pressing CTRL while dragging the mouse, which will select the "back" faces too.

You have the option to use the paint select mode which you click LMB in the first face and - while keeping it pressed - drag the mouse over the other neighbour faces.

If this is not happen, its because you can have the Tweak mode active. If so, you can disable it or do the same process but starting with a double-click (without release the LMB).

For what you need now, maybe to use the menu Select->Similar Normal [with options] can be a good way to get them selected. You should select one face and when you access the menu option you can adjust the Angle Tolerance which you will see in the preview the faces being selected.