Hold Object and place it by pressing a key


i want to create a tactic-shooter.
In the shooter you can run and kill people → you get money and with this money
you can build traps or towers(like Orcs must die).
So i want: If i press (example) b then appears a Gui
there i can select the buildings…
If i select want its in front of you (like Orcs must die)
then you can click and its build.
Thats the idea.
Now i want to know how i can do that???
pls help me!

(sry for bad English)

If press ‘B’ {
show GUI;

if click item in GUI{
   Create object in front of the player and them parent it to the player;

if press 'Drop item button'{
unparent item from the player

I think thats it.