Midjourney is definitely the easiest to get started with, you just join their Discord - https://discord.com/invite/midjourney, go to one of the Beginner channels, and write a prompt using /imagine -
Stable Diffusion is also mildly popular - https://github.com/AUTOMATIC1111/stable-diffusion-webui, but it takes a little bit of know-how to setup. It does require some level of technical knowledge to setup and use, but you can also try to find someone's Google Collab notebook and run it off there (something like this - https://github.com/TheLastBen/fast-stable-diffusion) . It does not have the SFW restrictions of Midjourney, but requires much more fine tuning to get right.
Outside of that, there are tons of random AI art generation websites and it will be hard to pinpoint which ones will match your needs, but these are probably two of the most popular atm.