When most plants first sprout out of the ground they generally have a set of leaves that look nothing like what an actual basil or parsley leaf may look like. These are called "Seed leaves" and are just there to power the plant until it is large enough to grow "True leaves" (which look much more like the plant you are expecting to see).
The general rule of thumb is that you should wait until your plant has 3 sets of "true leaves" before you cut anything. Once you have these 3 sets of true leaves, you can trim your plant down by cutting off the top section of the plant's main stem with sharp scissors at a 45-degree angle. Try to give the plant 5-10 cm of space between the light and the top set of leaves.
Some plants such as basil can be harvested repeatedly, and actually grow faster with a healthy level of harvesting - whereas other plants such as lettuce are single harvest and are best harvested all in one go.
Note: You can trim plants precisely and quickly using these.