I Brought a 16GB Lexar micro sd card for my LG Optimus L7II and installed the micro sd card and I've have tried downloading apps but it keeps coming up insufficient storage space available! How do I get the SD cards to work?
What version of Android do you have? Depending on the version (2.3+ I think) you can move apps to the external SD card via App2SD (or check the Settings/Applications for the option). You will have to install on internal first and then move to external, so to free memory you will have to move some other apps which are already installed.
But some later versions of android no longer support this.
If you root your phone, then try Link2SD.