To download an app, simply open the Play Store and browse through the various categories or search for a specific app using the search bar. You can also check out the recommended apps or top charts to discover new ones. Once you've found an app you like, click on it to open its page.
The app's page will show you its description, screenshots, and reviews from other users. Take a look at these to get an idea of what the app does and whether it's any good. If you're happy with what you see, click the "Install" button to start the download process.