Purchase a book of scales for singers. You can find vocal exercise books at a music store. Singing scales is one of the important things any vocalist can do to strengthen vocals and help develop breath control. Sing the scales daily, in different keys, both ascending and descending. For falsetto, remember to sing the opposite of what singers are told. Falsetto is like a head voice; it doesn't come from the diaphragm. When you sing your scales, remember to sing more from the vocal cords. You can place a hand on your chest to feel for vibration. If you're singing proper falsetto, you will not feel vibration in your chest.
Sing a classic yodel exercise. A yodel starts out in normal register but shifts to a falsetto near the end of the yodel. Yodel exercises can help you strengthen your falsetto because you force yourself to push the notes out as you shift from normal register to falsetto.
Continue practicing scale tones, but replace the notes with alternating patterns of vowels. Ensure the outer edges of your vocal cords vibrate as you sing vowels. Check this by keeping your hand on your chest and shifting back to falsetto when your chest vibrates.
Sing falsetto with any song that comes on the radio. Turn the radio up enough that you need to force your falsetto to be heard along with the vocals on the radio. Try to harmonize correctly while keeping your voice strong. Although a falsetto will never be as strong as a natural voice, exercising it regularly can help increase the intensity of your falsetto, and your improved breath control will allow you to project your falsetto voice to give the impression of more power.