For the purpose you describe, you can get soft glass tubing, heat it in the middle over a propane torch or a gas burner, and gently pull the ends apart. The tubing will shrink in diameter where it is melted as you draw it apart. If you draw it apart quickly enough, you will get a long thin tube of glass, similar in diameter to a hypodermic needle. It will be fragile and easy to break, but if you break it off where it is strong enough, and thin, but not too thin, it would be suitable for injecting a liquid into a plant. However, if you merely cut off a flower, and put it in a vase with the appropriate chemicals, I would expect the chemicals to get to the flower by capillary action and osmosis. This is commonly done with dies on various flowers to change them to interesting colors. If you have an adult to help (perhaps your science teacher) you can buy hypodermic needles that diabetics use at a pharmacy without a prescription.