I want to say that both "take on" and "take up" could be correct here depending on the nuances of what you want to say. "Take on" could be interpreted as a metaphorical way of saying "I want to challenge myself with a new hobby" while "take up" would just be "I want to begin a new hobby." In general, I think people say, "I want to learn a new hobby" more often than "take up", or you could also use "I want to find a new hobby to learn", if you didn't know what hobby you wanted to start yet, just that you wanted to start one. Anyways, these are just my unsolicited linguistic observations as a native English speaker that may or may not be accurate depending on which dialect, specifically, you are speaking. Hope they're helpful nonetheless.