If you like epic fantasy, start with 'The Lord of the Rings'. It's a classic and will introduce you to a huge and immersive world. For science fiction, '1984' is a great start. It's short but very impactful.
For science fiction, 'Foundation' is a good start. It has interesting concepts and a well - developed universe. In the case of fantasy, 'The Hobbit' (which is related to 'The Lord of the Rings') is a great introduction. It's a more accessible story with a single main adventure that can ease you into the world of Middle - earth.