Simple webpage on React.js
Great choice, Allan and good effort! React is awesome and fun. You can improve a few things if you are interested:


Separate Presentational and Container Components. This way your app will be easier to scale, read and maintain
You can try to have your state in one parent component and pass it as props to its children
Extract all components in different files
Navigation can be done with React Router. It is very simple to set up and easy to work with
Consider learning and using Redux for your React App state when you are comfortable with the basics or React
Enjoy learning :)