Here's some guides to installing Stable Diffusion (if you have a NVIDIA GPU with 2GB+ VRAM).

Installing AUTOMATIC1111, the most feature-rich and commonly-used UI:

Different UI with 1-click installation:

https://github.com/cmdr2/stable-diffusion-ui

Installing various UIs in Docker:

https://github.com/AbdBarho/stable-diffusion-webui-docker

AMD isn't well-supported. If you're running Linux there's a way to install AUTOMATIC1111 but otherwise features are very limited:

Native: https://rentry.org/sd-nativeisekaitoo
Docker: https://rentry.org/sdamd
Onnx: https://rentry.org/ayymd-stable-diffustion-v1_4-guide

To try it out without installing anything you can use a web service, though they are lacking in features.

Stable Horde is a free network of people donating their GPU time, how long it takes depends on how many people are using it. It supports negative prompts, seperate the positive and negative prompts with "###":

https://aqualxx.github.io/stable-ui/

Dreamstudio requires making an account and gives you around 200 free images before asking you to pay:

https://beta.dreamstudio.ai/
Img2img: https://huggingface.co/spaces/huggingface/diffuse-the-rest | https://dezgo.com/image2image
Ippainting: https://huggingface.co/spaces/fffiloni/stable-diffusion-inpainting | https://inpainter.vercel.app/paint

The most notable non-Stable Diffusion generator is Midjourney, which tends to be nicer-looking and doesn't require as much fiddling with prompts but can have a samey style. However the only way to use it is through a Discord bot.

https://www.midjourney.com/home/

If you're okay with more setup but don't have the hardware you can use a cloud-hosted install:

Paperspace: https://rentry.org/865dy
Colab: https://colab.research.google.com/drive/1kw3egmSn-KgWsikYvOMjJkVDsPLjEMzl

Various other guides:

NovelAi: https://rentry.org/sdg_FAQ
Dreambooth: https://github.com/ShivamShrirao/diffusers/tree/main/examples/dreambooth
Inpainting/Outpainting: https://rentry.org/drfar
Upscaling images: https://rentry.org/sdupscale
Textual inversion: https://rentry.org/textard

Resources

Search images and get ideas for prompts, can search by image to see similar images: https://lexica.art/
Index of various resources: https://pharmapsychotic.com/tools.html
Artist styles: https://rentry.org/artists_sd-v1-4 | https://www.urania.ai/top-sd-artists | https://rentry.org/anime_and_titties | https://sgreens.notion.site/sgreens/4ca6f4e229e24da6845b6d49e6b08ae7 | https://proximacentaurib.notion.site/e28a4f8d97724f14a784a538b8589e7d
Compiled list of various models, regularly updated: https://rentry.org/sdmodels (Check here before asking "where do I find x model!")