Before you start and download applications that are fairly big (like the one in your case), it's advised to make sure that you use a stable WiFi network connection. Due to the fact that larger application need more time to download, any disconnection from the network could lead to errors. To fix your problem, try going to Menu > Settings > Application > Manage Applications > All tab > and tap on the Google Play application. From there, your will need to Clear Data and Clear Cache. If you didn't manage to solve your problem, try to reboot the device. This could also help because by doing this, all its cache memory will be cleared.