Answer: You didn't specify a plant, so I'm not sure how to tell you to pinch it back, but the general rule of thumb is to pinch out the growing tip to make the plant develop new stems. This will make it bushier. You wouldn't pinch out the tips of single stemmed plants such as sunflowers, but you can pinch out the tips of branching plants such as marigolds and chrysanthemums.
If you're starting plants from seeds, you'll want to remove those that are not thriving. Rather than pull them out of the soil, which can disturb the roots of other small seedlings, try simply cutting them off with a pair of scissors.
Hope this answers your questions!
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