Grapevines need at least annual pruning if you want them to produce a good yield of grapes. Grapes grow from the one-year-old wood on a grapevine, so older wood can usually be pruned away, as can any that is unwanted. Once-a-year pruning is usually sufficient for most grapevines, although occasional maintenance pruning may be needed after a windstorm, for example, to remove broken parts, or if the vines are attacked by disease.
Prune at the right time, when grapes are dormant. This means in the winter; for warmer climates, January through March is the best time, while gardeners who have colder, longer winters can prune in a wider window, from November to April.
Train first-year grape plants vertically until they reach a string, wire or pole to grow laterally upon. Throughout the growing season, prune off any horizontal runners or shoots so that the young plant grows straight up, forming a trunk. Once it reaches the desired height, select two opposite horizontal shoots near the wire or support, and train them along the support to make a "T" shape. Remove all other shoots.
Prepare these two chosen "canes" to be the fruiting shoots of the grape plant the following year, by continuing to train them laterally along the wire or support. The next winter, prune off the newest growth and many of the buds they produce, leaving seven or eight of the first buds on each cane for future grape production.
Thin out fruit clusters in the third growing season, which is when the grape plant will produce fruit. Each of the buds you previously left will produce a downward shoot with grape clusters; take off all but one cluster on each shoot to keep them from over-producing.
Choose an ongoing pruning method, either cane pruning for French-American hybrids and some American grapes, or spur pruning for most other kinds. Cane pruning will mean you choose which fruiting wood to retain, and cut the rest off. Spur pruning means you cut back all the canes to just a few buds, and let them all grow again. Choose which is the best for your variety.