Quick answer
Я понима́ю.
ya panimáyu — понима́ю for ongoing understanding, по́нял for “got it”
The options
понима́ю or по́нял?
Понима́ю is the ongoing state — I follow you, I understand the language. По́нял is the perfective past: the moment the penny dropped. In answer to an instruction, Russians say По́нял, never понима́ю.
Поня́тно is impersonal
Поня́тно has no subject at all — “it is clear”. It is the most common of the three in conversation, and Всё поня́тно means “that explains everything”, sometimes drily.
Gender in the past
The perfective past carries gender: a man says по́нял, a woman поняла́ with the stress on the end. Plural is по́няли. Getting this one right is worth the effort because you will say it constantly.
What you hear back
Typical mistakes
Понима́ю! — in reply to an instruction
По́нял! / Поняла́!
A single act of grasping is perfective.
Я по́нял — said by a woman
Я поняла́.
The past agrees with gender.
Check yourself
A woman replies “got it”: