U may go to file manager then sdcard0 then whatsapp (if u do use it) then media n there u have images audio n video which generally blocks up a lots of space if not cleaned. So clean the folders if u have nothing important inside there.
U may also try uninstalling apps tht u dont use from the list of apps by pressing the options key on the left bottom of the handset and there uninstall and then clicking on the apps u wish to remove n then ok.

 

I dont know exactly regards that but i think texts are never an issue as far as space is concerned as they take vrry less space. I think they might be on phone memory. If u wish to check available space on ur phone and utilised by what u may press the left bottom button on your phone when the wallpaper is visible and then settings and then storage. There u get it all.

 

Its the data on the phone memory, not the SD. So, manage apps/ look at what is taking up loads and try deleting cache. That will come back though.