There isn't any one "etiquette" to using them. Different users use different reactions to different posts.
Personally, I think the reactions are mostly self explanatory. I use agree if I agree with someone (sometimes also used if one has had a similar experience), informative if I learned something new, funny if I found something funny, creative if they are showing off nice art or stories (though I personally also use it sometimes when people use creative argument, or look at things from a unique perspective) and helpful if they are giving advice or talking about personal experiences which could be useful for others (or the person asking for advice).
Friendly is generally used when new users join and introduce themselves (such as you), but also when users are talking about tough times and struggles to show sympathy. I also see it used if someone disagree with what a user writes, but they don't want them to take it personally when they rebuke them.
I don’t use like much, but I sometimes use it if I "like" the general contents of a post without it falling into other categories. However, some other users use like much more frequently.
I use optimistic to show that I hope for the best when people are dealing with turmoil in their life, or are having to make tough decisions and be decisive. Also if people are taking job interviews, getting therapy or seeking out a diagnosis.
I tend to use "winner" either if someone had a major success in their life, and are doing well (say, getting work, made a tough decision that worked out for them, literally won at a competition), if they make a very good argument, or if they simply say something profound, which I think really "hit the mark", so to speak.
Sometimes it can be tough to pick, but ultimately it's judgement calls and reactions don't matter all that much. For instance, if I read someone else write this message, I might potentially give an "informative" (because I'm learning how they use reactions), "agree" (because I use reactions the same way), "helpful" (because I'm answering your question), "like" (because I like the substance of the reply) or "winner" (because of the thoroughness of the reply). That's why you normally get a mix of reactions to a post.
On a final note, these are general thoughts on how I personally use reactions on most posts. However, I also often give reactions humorously (for instance, using "informative" when someone is obviously making a joke) and sometimes people ask for specific reactions (like one of each type), so I don’t think you should give yourself hard rules on how to use them. Some users don't use reactions at all and see them as pointless or anxiety inducing!
Welcome! As you can see, you have joined an autism forum.
