Hi Meryenn!
Welcome. My mom's turtle finally passed on this year after 45 years! It's amazing he lived a full life considering how we had him housed most of that time. We got him in the 70s and had him for years in a cheesy small plastic dish they used to sell in pet stores with a fake palm tree in a raised area in the middle. We eventually moved him into a 10 gallon tank, way too small, no heater, no filter, no lights except sunlight through the window. After many years we moved him to a 20 gallon long (still way too small) with a filter and heater, but still no lights. Then finally thanks to the internet, I upgraded my mom to a 40 gallon and he finally had the set up he should have with suitable lights, filter, heater, thermometer, basking area, etc. Still a little too small since he was 7 inches by then, but the best we could do.
I really can't believe he lived as long as he did!
Anyway, welcome. The best way to tell male from female is to look at your turtle's front nails. If they grow really long, it's a male. It has something to do with them stroking the female during mating. We always thought ours was female until we read this; his nails were super long. This page has a good picture of it:
https://kamekroten.wordpress.com/2011/1 ... or-female/