Unable to bring up mac desktop on my moniter - Help!

So, hello everyone, first time here, be kind! :-D

I have a macbook and iv kindly been given a dell moniter (too skint to buy a new one). Iv hooked it up using a moniter cable and VGA adapter so it fits into my macbook miniport. However, when I connect the two, I get the screens flashing as par all the online tutorials iv seen on utube, then er nothing? The moniter goes black. The only time I see the desktop on there is when i switch it on and off and it does not stay on there.

I woudl love some help, is there a preference I have not set on my mac?

Thanks so much

Kind Regards

MissD