In need of help of being able to download

[aa122]

I just bought a new memory card so i can download stuff, i don't know how to make it work to where i can download things every time i try to download a game or something it says " error".

 

[Calculagator]

The solution could be a lot of different things. It will be very difficult for anyone to help you unless you give more information:
What phone are you using?
What version of android are you using?
What app says "error" and is that the exact message?