Why can't I sculpt on a plane I added a solidify modifier to? So, what I've done is to model a dagger using a plane. 
And to that plane I've added a solidify modifier, however, when I try sculpting it using one of my brushes it only turns into a disfigured piece of junk. 

How can I solve this? 

I saw a video from a few years back and it looks like that person is scaling the plane, but when I try the same thing it doesn't work (hence why I added the solidify modifier).

I'm a blender noob so don't be afraid to become too detailed in your answer.
You have two options:

Option 1:
In sculpt mode, on the left tool shelf you need to enable "Dyntopo" the lower the number the more detailed your sculpt will be. 

Option 2:
In the modifier panel add a subdivision surface to sub divide your plane so there are more faces to sculpt. 

Hope that works.