Weeds, if left to grow, can directly contribute to your lawn’s malnutrition. Weeds like crabgrass and dandelions are quick to absorb nutrients, and a lot of them, leaving almost none for your growing grass. In order to keep your grass alive, you’ll need to be diligent with weed removal.
Chemical-based weed products are toxic to humans, animals, and other plants, not to mention generally bad for the ecosystem. Instead, choose a natural option to kill weeds. Vinegar can be applied directly to the weed leaves to kill them. Just be sure only to spray the weed and not your whole lawn. Techniques like pouring boiling water on weeds are also effective. Some have found success using corn gluten meal as a natural herbicide—when mixed with soil it can prevent new weed growth—but the results may be inconsistent. When in doubt, you can always dig out the weeds directly (which may be more effective for very deep-rooted dandelions).