Yes, you can visit Pompeii freely without a tour guide, you could use an audio guide or a printed guide, or even use the information you find on the web. I recommend, if possible, booking an experienced tour guide in advance who will give you basic information about the life of the inhabitants, art, religions and everything that happened in Pompeii. A guided tour lasts about two hours, after which you will be free to wander around the park to deepen what you have heard and see places that the guide did not have time to show you. I recommend booking the guided tour a few days or weeks in advance, at certain times the excavations of Pompeii are very crowded and groups may already be full.