AI means a million things. In mid-2025, most people are talking about ChatGPT and its competitors, image generators (Midjourney, et al), and more recently video generators. For starters, lets focus on the chat interfaces, starting with ChatGPT.
I recommend a paid version of ChatGPT (the "Plus" level) if you are planning any consistent usage. It gives more bandwidth and more features (I say bandwidth as there is not really a good term for the amount of usage you get).