"I am sitting."
Translation:मैं बैठा हूँ।
बैठना is usually used as a stative verb to refer to the state of being seated. However, the continuous form 'बैठ रहा' is used for the action of sitting down. So, 'मैं बैठा हूँ।'=I am seated/I am sitting and 'मैं बैठ रहा हूँ'=I am sitting down
खड़ा(stand) is a different beast altogether. It is not a verb but an adjective in Hindi. If you want to talk about the action of standing up, you need to combine it with the verb होना (to be)
बेटा = son बैठा = form of to sit
Differences: first vowel sound (be vs bai), and aspiration of the second consonant (ta vs tha).
Probably to most native English speakers (for me anyway) the vowel difference is easier to get right as a distinguishing feature. Aspiration is hard without really exaggerating it and probably sounding quite silly.