Summarize this article:

If you purchase a new grape plant, you will likely find that it has a dense root system and many shoots rising out from the top. Immediately before transplanting the plant into your garden, you should cut the shoots off, only leaving the strongest intact. Look at the buds on your remaining shoot. Count off the lowest three buds, and cut the shoot back just above the third bud. After you transplant the grapevine, new green shoots will begin to form. Once these shoots reach a length between 8 and 12 inches (20.3 and 30.5 cm), choose the best one and remove all the other shoots. The shoot should be strong and nearly upright, and it should also come directly out of your original stem. Do not choose a shoot coming out of the underground root system. Support your remaining shoot by tying it to a stake or fence. Tie both the top and bottom of the shoot.  Throughout this first summer, continue tying your shoot to its support system, keeping it as straight as possible. This shoot will act as the permanent trunk of your grapevine and will last throughout the duration of the vine's life.

Summary:
Reduce the shoots. Cut back the buds. Remove all but one shoot after new shoots start growing. Maintain your main shoot.