Hi,
Have you tried updating the Google Play Store app?
Open the app, tap Settings, scroll down to Build Version, tap to enter. If there is an update option select it and allow it to update. Try to download an app and see if it works.
If it says it is already up to date, try the following: From Main Menu on phone go to Settings > Apps > swipe right to All > scroll down to Google Play Store, tap to enter, tap ''clear data''. Try to download an app. Hopefully this may help you.