American English speakers use 'i guess' as a mildly positive acknowledgement or agreement after someone has made a statement or asked a question. "Do you think this is ok?" "I guess". Whereas British English might respond with "I suppose so". In British English guessing is more reserved for uncertain responses or an "educated guess" where the person is allowing a fair degree of possibility they are wrong and are making a statement based on intuition. "I think" is generally more certain than supposing or guessing something. Starting with "I think" would usually mean someone is about to express an opinion or make a statement they believe to be true.