Hi woodyboy,
It's easy just play a 501 game count how many darts it takes you to finish it divide 501 by the number of darts thrown then multiply by 3. If you are playing a match against someone and lose the only thing different that you do is subtract what you have left for a score from 501 then use that number and divide by darts thrown x 3. Hope this helps you..but, My advice to you though is at 13 years old do not put much value on your avg. it can get frustrating, and averages do not translate to wins and losses I have seen many low avgs. win matches it's not the avgs. that win it's wether or not you can hit your outs.
Cheers,
Hatrick