Looks like a juvi murray shortneck
For food, shortnecks are more omnivores (in comparison to long necked species) and will eat aquatic vegetation and some aquatic animals, such as shrimp, guppies, other fish etc. You can buy live feeder fish really cheap from most pet stores, and you can also purchase aquatic plants from there as well pretty cheap usually- I just ask for the hardiest, quickest growing plants, because my boy loves to eat them
As they get older, they tend to eat more plants. I also supplement my boy with some nutrafin max turtle pellets and/or reptomin pellets, but the more natural you can keep the diet the better.
To sex the turt, have a look from the underside and look at the position of the cloaca, the width of the tail and the length of it. Females generally have sharply tapering short tails with the cloaca nearer the shell, whilst males have longer, wider tails with the cloaca further towards the end of the tail
An easy way is if your turtle is a male and happens to get 'excited' when being picked up and 'flashes; you lol.
Make sure you have a good basking area a few degrees warmer than the water temperature, and you'll need a good UVB (I used a 10.0 bulb). I don't use gravel or sand in my tank, because I've read about other peoples experiences with it, where the turtles have eaten it and been unable to pass it. Instead, I like to use the largest size polished river rocks from bunnings (cleaned very thoroughly) as my boy likes to push them around etc. but cennot swallow them. And I'm sure you already know this, but turtles create a LOT of waste, so you'll need a large canister filter to cope with it
