It really depends on who you're talking to. I'm a guy, so complementing other guys can be tricky without coming off sarcastic or weird. Women are super easy to complement because most women spend a lot a time on their appearance. If you're just trying to be friendly and not come off as flirty then complement them on their hair or clothes (avoid complementing on short skirts and low cut shirts and things like that because then you'll sound like you're checking them out. Unless that's your intention). This is one I said a couple nights ago when I was hanging out with a bunch of friends and a girl that I don't really know was there. I said "That's a really, really cool jacket. Did you notice if they had it in a men's large?". I got a laugh out her and she definitely appreciated the complement. It was a really cool leather jacket by the way, so it definitely came of as genuine. I think guys are a little trickier because a lot of guys don't care as much about what they're wearing and their hair and stuff like that that's obvious. If a guy is dressed well and you can tell he tries to look fresh when he goes out, then definitely complement on shoes/sneaker (especially sneakers if the person is wearing trendy sneakers like Jordan's or Sk8-his or something like that). If you're not really into men's fashion at all, I would recommend looking through WAYWT posts on /r/malefashionadvice to see what's trendy and to find things to complement guys on. If you come across a guy or a girl that doesn't seem to care about the way they're dressed, or if it's in a setting where people don't dress well like community service or a hike or something like that, then you need to dig a little deeper to find something. That's when you have to ask questions and find what people are passionate about. So if you find out someone's in med school you can say something like "wow, that's really cool! You must be a super motivated person to be able to handle that work load, good for you!". Just try to be yourself and sound genuinely interested in what the person is talking about. Don't over do complements either. Just 1 when you meet someone is enough to get on someone's good side. More will seem like your not being genuine that your just doing it so the person likes you, then the complements won't mean anything and you'll be worse off then when you started. Just try to sound genuine and like you actually like that person's sneakers or you are actually impressed that that person is in what ever field.