Do you have it growing up a trellis or arbor? It most likely needs to be properly pruned and tied during the dormant season. Generally, grapes are trained to a couple of strong canes, from which new growth springs every year. You would want to take a look at some pruning diagrams to get a better idea of how this works. If you do any major pruning other than during mid winter, the vines may bleed quite a bit.

For this year, pinch back extra shoots that develop after you can clearly see what grapes you are going to get. Also, cut back leaves that are shading the clusters of grapes.

Here's a very informative pub written by Oregon States's grape specialist
http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/21285/ec1639.pdf

Here is a link that might be useful: Growing table grapes