1. मुझे हिंदी आती है (mujhe hindi aati hai)
This is a more literal translation of "I know Hindi" and is widely understood.
2. मैं हिंदी बोलता हूँ (main hindi bolta hoon) / मैं हिंदी बोलती हूँ (main hindi bolti hoon)
This translates directly to "I speak Hindi" and is also commonly used. You would use "bolta hoon" if you are male and "bolti hoon" if you are female.
Both phrases are grammatically correct and convey the same meaning. Choose whichever one you feel most comfortable using.