1) Consider if your parents make homophobic comments or whether you are financially dependent upon them, etc. If any of these questions were a yes, then its probably best to wait until you're independently living or have a good support system.
2) Slowly drop hints to see how they react. Say how your friend came out gay and mention how proud you are of them. Look for your parents reactions etc.
3) If you have relatives, friends, teachers or counselors who already know you're gay, establish a support system with them. They are someone to help you out so you dont feel alone in case your parents react badly
4) Have a backup plan if the conversation goes badly. If your parents cut you off financially or ask you to leave the house, you need to have somewhere to go and someone to help support you.
5) Figure out what you want to say, starting the conversation with the hardest part. Make sure to sound serious and let your parents know it's important to you. It's okay if you're scared.
6) Explain to your parents you were sacred or why you havent told them sooner. This will be a big assistance in helping them to understand you
Disclaimer: This is a just small guide meant to help and guide you due to that its not always guaranteed. Remember to take this with a grain of salt and consider what works best for you not all of these will apply but are just mainly points to consider.