Kelly is right about "Guess what". You just say that to tell someone you have exciting news to tell them (if they want to know, they'll say "What?" in reply). "Guess it" can be interpreted literally.

Guess what is a idiom. You say Guess what? when you want to tell someone something surprising or exciting. Guess it. I don't know it for sure. I think it's better if you can put it in the context and I'll say what this mean. But I think you say Guess it when you want someone to give you their guess.

1. 'Guess what?' is a kind of discourse marker, used in conversation or informal writing, to show that the speaker is going to reveal something interesting or surprising. 'Guess what? I won the won the competition!' 'Guess what?' Joe and Sally are getting married. You can also extend to make a longer question. For example: 'Guess who I saw yesterday?' You're not really asking the other person to guess anything - it's just a way of getting their attention and signalling that you have something interesting to tell them. 2. 'Guess it' isn't actually a common phrase at all among native speakers. I can't think of a natural situation when we would say this. If you genuinely want someone to guess something, then you just say 'Guess!'. A: So, what mark did you get in your exam? B: Guess! A: I don't know. 80%? B: 100% !