If you have the apk file (app installer) for the app:
1) copy it to your phone's sd card
2) enable "install from unknown sources" in your app settings menu
3) select the apk file with a file explorer app (try ES File Explorer if you don't have one). Clicking on the file will open the install dialogue.
Otherwise, turn on WiFi (or data) and visit the play store (play.Google.com). Log in using the Gmail account you use for your phone, choose an app and select "install" and it will download and install on your phone.