Just adding an SD card to your device isn't going to do anything. You have to go through your old texts and emails, and either move them to the SD card if it will let you, or delete them altogether. Go through your apps and move as many as possible to your SD card. Uninstall any apps you don't use regularly (and be ruthless). Move your contacts and images to the SD card.
And be aware, the 16 GB you see for memory is your internal memory. You have to look at the external memory to see the SD card.