The best place to start is to not have crud accumulate on your backdrop. That's usually as easy as cleaning off the bottoms of their shoes (and yours) before they step onto it. I'd say give it a good wash, and try to keep it relatively clean going forward.
By the way, that's the main reason photographers go with rolls of seamless instead of cloth backdrops when they can. When the seamless gets dirty, you cut off the dirty part and throw it away, and pull out a few more feet to replace it.
In the meantime, cleaning such things in Photoshop depends on how much crud there is, which is why I suggested you post an example photo. It could be simple, or more difficult, depending, and that could depend a lot on your Photoshop skills. Since you're asking the question, I'm going to guess that they're not very advanced yet.
Techniques range from using clone, heal, and patch tools, to copy and pasting sections, to painting it with a brush, to cutting the subject out with a good mask and dropping in a new area in to replace whatever has the crud you want to get rid of, or combinations of all of them.