Drag the decal tool on the surface you want to "paint"
select a texture in the properties... then you can adjust the color, even the size and many others properties.
There is many many options then for more complex adjustment. I think you will have more explanations with upcoming tutorials.
Hello Kushland, how you doing buddy? would you be able to send a pic of your issue? try this - go into the properties of the decal and try moving the Decal Target to Custom, then bellow that setting move the Target Mode to Inclusive or Exactly, if this does not show anything then try going into the wall or object you are trying to put the decal on then go into the properties of the selected object ie. the wall - go down to the Decal Target Category and move that along to custom. Hopefully your decal will now be showing on the object.There are many combinations using the settings involved to get decals showing up on certain objects using the decal options and object options.