The problem is that even though you have an 8GB memory card, the internal memory on the phone is about 1GB, this is a type of system memory and many apps require some of this space. You have managed to fill the system memory with apps and that is full, not your memory card. There are two solutions.
1. Delete some apps to free up some of the phone memory!
2. "root" your phone and use an app to force things onto the sdcard instead of system storage. You can google how to root your phone and there are plenty of apps that move things to sd card.