Hi! First of all how tall do you want your lemon tree to be? If you want to keep it in a ball shape for example and do not want to have to use a 12ft ladder to trim it just let it grow to 6ft.You can have a wonderful 6ft lemon tree with a great huge ball but you need to let it grow,grow,grow,keeping that general shape without cutting it hard until the tree has grown to the desired height.
Good Luck

Took this from an article which describes it far better than I could.

Step one  Prepare the shrub. Once a year in spring for the first couple of years after planting, trim the side stems back by a third. Meanwhile, let the leader  the tip of the shrub  grow. Leaving these trimmed side branches attached helps strengthen the trunk and supply food for the plant as it grows. When the leader reaches the height you want for the center of the ball, pinch back its tip. In other words, if you want a 4-foot-tall standard with a 16-inch ball, let the leader grow until it's about 40 inches tall, then pinch it. You can let the standard grow to any height, but I find two-thirds trunk, one-third ball makes a nicely proportioned tree.
Step two  Create the trunk. The spring after pinching, completely remove all the lower branches on the main stem up to what will become the bottom of the ball. (That would be 32 inches in the previous example.) Pinch the tips of the remaining branches so that the ball "bushes out." Then let them grow for the rest of the summer.
Step three  Make a cutting guide. Tie string to the stem at about the center of the ball to be. Pull the string taut, then measure half the planned diameter of the ball (8 inches in our example) out from the leader. Mark the string with colored tape or ink.
Step four  Form the ball. Keeping the string tied to the same spot, pull it taut and clip all the stems extending beyond the mark on the string. Rotate the string up, down and all around the main stem until you've created a ball. Because some branches might be a little shorter than others, it sometimes takes a few months to develop a good, tight sphere. Just keep the longer stems clipped back to the guide and let the shorter ones catch up.