You need find out what species it is before feeding it too much of random human foods. You can/will make it sick, or poison it to death by not feeding it properly, so its really important to find out what it is.

Softshells are known to be carnivores (includes bugs) & might eat some vegetation.

here is some general information on a Softshell's diet... but its still not a proper diet until you find out what species it is.
http://www.ehow.com/way_5744255_soft...rtle-diet.html