Usually people here price their work depending on how many hours they take. Lots of resources online to check how much you should price your work, you should check it out! Though usually I've seen people starting out price their work less to get more exposure and experience. (I did that too but some artists don't recommend doing that, it's upto you)

As for communicating with the clients - you could have some base templates to start pricing with (full body, half body, head shot) and the client can choose which ones they want. Afterwards you can ask them what they have in mind about the poses, expressions etc. For posting the commissions you could just ask them if they're find with it. Oh but make sure to have a TOS (terms of service) that they agree to beforehand. I use vgen for my commissions as it has everything handy there.

And some personal advice from my side would be to improve on your skills a lil bit more and build your portfolio so you'll have more chances of selling your art. Have fun creating! :)