These are the things you must know.

I get these questions a couple of times so now you can read this.

DELETING PROPS
There are 2 ways to delete props:
Using the remover tool - press Q and click on remover tool. gone prop!
Pressing Z - pressing Z makes you undo.

MAKING A GOOD BOX
You can use a lot of tools for this but I do the following
Use the easy precision tool - press Q and click on easy precision, the select the according mode and go ahead.

MAKING THINGS INVISIBLE
you can use the color tool for this.
Usage of the Color tool - press Q and click on color, you see 4 boxes, all with a number between 0 and 255, the bottom one is the opacity, put it to 0 for invisibility. NOTE: don't color a light as it won't light up any longer.

Have fun with Garry's Mod