ok
so to start you need to know some bassic tips like how to place a cube, to place a cube you go to GameObject (top left) select 3d and selcet a cube, if you dobleclick on the cube in the hiearchy it will center on the cube,
another tip is to know the x y z colors x is red y is green and z is blue, in the right side of unity, if you click on the cube in the hiearchy you will see the cube "stats" like his position in x y z and the size of it, if you want to make a 10x10x10 cube, all you need to do is to change the cube scale to 10 in x y and z, another tip is to first thing when you place the cube it wont allways be in the center of the game so in the x y z location change everything to 0,
if you want a 10x10 platform you just change the x and z to 10 cause y represents the hight, and if you want it to be thicker you can change the y to (what ever you want) something like 0.5, that will make it half as thick (btw i lerned all that in one day)