I fear some jump into it having only seen 50 Shades of Gray.

 
I am not a real big fan of BDSM, but some gals I've seen as GFE revealed to me that they also do BSDM as doms, and so  being curious, I engaged with them in it and found some things I liked.    I also met a gal who liked to receive abuse in the form of slapping her across the face while we had sex.   This brought her to orgasm, but left me feeling kind of weird.  She kept insisting that I slap her harder and harder to the point where it left noticeable marks.

 
As for pain, start off on the low end and build up to where they tell you enough.   I had gals do this with lashes and riding crops on my back, and found it quite enjoyable.   Tease and denial is also a lot of fun.  Of course, not BSDM session (at least for me) would be complete without her ordering me to pleasure her in specific ways that get her off.  I also tried out being bound with fur lined cuffs and blindfolded, but that didn't do much for me.   One gal had me bind her legs and arms to the posts of a four-post bed frame using nylon stockings, then cut her clothes off with a knife as foreplay.   I liked how it got her excited, but again weirded me out.

 
This is, admittedly, "vanilla" BDSM.   The hard core gets into all kinds of iffy situations involving whips, chains, being bound and even playing with asphyxiation.   You'd do well to hold off on this kind of thing until you get a PhD from some highly skilled practitioner.

 
Best of luck to you in getting into this exciting field.

That there are more men seeking doms than there are doms.  I can't recall where I read that though.  But I think there is a market for it.  
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I think you have to run the show.  So a major factor is developing a repertoire of scenes for many different tastes.  I believe that is the hardest part ... developing your act(s.)   Clients are all going go want different focus and different levels of punishment and humiliation.   It's actually a lot of work (at least at first until it becomes more second nature from experience.)
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Also besides a lot of lessons on youtube, you can peruse sites like fetlife, for fetishists and their playtime activities.

Lots of ?’s

1. No. Self taught, but I have done duos with Dom Dominque.
2. You need a good bag of tricks for starters & you will build your equipment as you go. Many gents will pay for toys & you keep them.
3. All my favorites, do not write reviews. BDSM is not valued here on TER. I am a lifestyle Domina & my reviews are GFE. The highest score you can receive for BDSM play without engaging in Sensual play is lowered. Most guys don’t with to share that they love being pegged, or enjoy golden showers is still taboo on a mainly vanilla board.

#1 Pain should be a part of sensory enhancement, not an end in itself. For me, moderate is right if it is mixed with pleasure.
#2 yes, yes, and yes. Plus bondage ropes, cuffs and collars. Bondassage is great and you should consider exploring that.
#3 no reviews because the dom didn't want any. But I would be happy to give you more insight in a PM if interested.

I suggest you watch a new documentary video that just came out about a real Dom, Mistress Snow, and a real depiction of  the life of a Dominatrix, https://wickededen.com/