The best way to prune grapevine for max fruit production is when the vine is dormant, before bud break. Where you live, this is best done in August, about 2 weeks before bud break. Without knowing the type of grape you are growing and the current training system it is difficult to provide very specific information but I will try my best and if you can provide more detailed information then I can try to help further. I found this document produced by the Government of Western Australia which provides lots of information on growing table grapes in the home garden. It can be accessed at the following link:

http://archive.agric.wa.gov.au/objtwr...

To summarize the document, the two basic pruning methods for table grapes are cane pruning and spur pruning. Cane pruning is used on varieties that produce fewer bunches (bunches on less than half of the newly emerged shoots). Spur pruning is used on very very fruitful varieties (one or two bunches produced on each shoot). Obviously the technique you employ is dependent on your grape variety. Can you provide this information? I will update my answer when I know some more.