At a young age, it's quite difficult to sex bettas for the untrained eye. Here are some tips though.
Males will tend to grow a longer anal fin before any of the other fins get longer iirc. Males have a more pronounced 'beard' (makes them look extra badboy while flaring). Their bodies are less wide and more elongated (similar to yours). Females have 'egg spots' between the ventral and anal fins. This is their ovipositor tube which drops during spawning, so if your betta has no white spot, it should be a male. Also, there are various behavioral differences which are quite prominent. Although I'm not sure if maturity is required before they exhibit a specific behavior - particularly bubble nest making and 'flaring' (females don't do it as much).
I hope that helps! Also, I'm sure the experienced Betta vets will come here and lay the smack down on me for my n00bness and provide much better advice.