From the App Store or Play Store, you can download and install apps for the Desmos graphing and/or scientific calculators. (Your phone/device probably already has a basic calculator app. Typically, basic calculators come pre-installed.)
You can also visit the web-based platform for all three calculators by Googling "Desmos" and selecting the type of calculator you desire.
For your convenience, you can choose the appropriate link below to get started with Desmos right now. You may wish to read the instructional documents for the graphing and scientific calculators to help you get started.
Link to Desmos Four Function (Basic) Calculator: www.desmos.com/fourfunction
Desmos GRAPHING
Link to Desmos Graphing Calculator: https://www.desmos.com/calculator
Desmos SCIENTIFIC
Link to Desmos Scientific Calculator: https://www.desmos.com/scientific