I would recommend taking a "pre-determined system" and simplifying it, because making a whole new one will not be an overnight task, to say the least. I do not have experience in tabletop roleplays at all, so take my advice as a suggestion if that is an indicator of validity.

Also, for dice rolls, you can google a random number generator and set parameters for the probability you want. On [url=http://www.mathgoodies.com/calculators/random_no_custom.html]this site[/url] you can set lowest limit(1) and highest limit(6) to make a d6, or lowest limit(1) and highest limit(20) to make a d20, so on and so forth.