Knowing any kind of programming language will make it easier to learn another one, for the simple reason that you will know all of the fundamentals.
How long it will take for you to make games is entirely up to you, if you have no games programming experience it might take a while, and you will first have to get used to the general feel of DarkBASIC, but games such as Pong or Space Invaders are by no means games that you have to be a pro for to make.
Professional-like commercial games is another story, most people here after a long period of experience still dont make it to this stage, but with the right amount of determination and effort nothing is impossible
Cheers,
Kentaree
I would be unstoppable if I could just get started...