The best advice is to simply start! The emotions that come with being in such a new situation might feel a little overwhelming, so take it easy, and don't add pressure on yourself by stressing about future sites, as just because a site works for you doesn't mean it'll work for another person, and vice versa. The experience you'll get from simply starting camming will be worth more imo.
SM is definitely a great starter site. It's gonna feel a little silly, but since SM is a site that has the option for members to type without others being able to see it (they'll have "invisible" next to their name), this means you can get away with simply talking to yourself as if someone's there, because how's anyone going to tell the difference between an imaginary member and a real, invisible member?
You can take this opportunity to talk about something like, say, "yeah, I can do that! So if you want to see me flash my boobs, tip x gold. I have more options in my tip menu" to encourage tips from real viewers. Sometimes I just say something quick, like "aw thank you!" and smile to stop there being radio silence; it helps keep my own mood up while real viewers have yet to start talking.
In my experience, I feel as if viewers are more willing to talk when the cam model is already talking.
You sound like you've done the research, but just in case: remember that you have the power to say no, that your boundaries are your own and you never have to explain yourself. Block if anyone is ruining your mood. And please remember that the first 30 seconds are free for a viewer on SM, so don't do requests right away/rush into them in that 30 secs. Asking a viewer what they want to see in the first 30 secs is a good way to spend the time if they haven't clarified already.
And remember that SM has a high pay cut, so don't be afraid to bump up your prices often until you find what works for you. My prices right now are 4.49 for private shows and 9.99 for exclusive, but those prices do not work on other sites (for me) unfortunately, so I was undercutting myself severely when I first joined SM. If there's something I'd do over with SM, it's that.
Good luck, and I hope you have a positive first day!