"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)
For most verbs yes, that's the form used for continuous, but a few verbs are unique, like sitting = मैं बैठा हूँ, and standing = मैं खड़ा हूँ, there are probably more, these are just examples.