In short, the way you use civitai is to find "checkpoints" (i.e models) which are mostly modifications of pony atm, itself a modification of Stable Diffusion XL. And then, use these checkpoints combined with LoRAs and good prompts to start generating. Other tools like ControlNet, inpainting etc become very useful once you understand the basics, and they take more work to master and understand.
Read properly the info shared on the page where you downloaded a checkpoint, and find pictures you like on civitai and download them as .png using the website's UI. Normally, you should be able to get the settings used to generate the image, and LoRAs by putting this .png file into A1111 for example, it can read its metadata and give you the prompt used, resolution etc.
You can find lots of tutorials on youtube for stable diffusion etc.
Make sure to use supported resolutions by the model you use. I believe these are the supported resolutions for SDXL. This should already help a lot, use any of the following (I recommend 832 x 1216 and 1216 x 832 to begin with, just because I like them) :
1024 x 1024
1152 x 896
896 x 1152
1216 x 832
832 x 1216
1344 x 768
768 x 1344
1536 x 640
640 x 1536
But now, flux.1 released and it's the best model out there, you can use it with comfyUI, it should be the best option. It released recently so there may not be as much documentation, LoRAs already made, etc. Than pony.
