Download/Install Android apps

Are you installing from Google Play Store? If so, it will only allow you to install apps compatible with your phone. Check the system memory in your phone. When apps try to load and there is not enough memory they will stall. Try uninstalling some of the apps you don't use often and then try installing a new app and see if the same problem happens. A system reset will bring your phone back to factory condition but doing that will lose all apps and data in the phone. Apps should be there to reinstall from the "cloud" from whichever site you got them from. Back up your data off the phone before doing any system reset. Google your phone to see exactly how to do a full system reset.