How do I sell bitcoin using the app?

Hello, I'm a beginner in the cryptocurrency market and I've just started investing in bitcoin. However, I'm not sure how to navigate the selling process through mobile apps. Could you please walk me through the steps involved in selling bitcoin using a cryptocurrency exchange app? Specifically, I'd like to know how to set up the transaction, choose the right selling price, and ensure the security of my funds during the process. Additionally, are there any fees or taxes I should be aware of before making the sale? Thank you for your assistance in this matter.