even creating stop-loss orders confuses me a little (like why someone would want to do that opposed to just a limit order?)

I should do a post about order types. These work totally differently.

Would you have a suggestion for where to start? A free-to-play demo account that still acts true to life? There are so many options I don't know who has the better show...

This depends on what types of assets you want to trade. A demo account for equities with a legit market feed is a very rare bird. A demo account for forex is really easy to get. Different asset classes behave a lot differently though, and which is appropriate for you depends a lot on your strategies.