I wouldn't sweat it too much. It's the sort of thing everyone needs to learn by practicing, that's how everybody who is any good at it got there.
If it worries you, maybe start with innocuous compliments, things like that whatever looks cute, you have a pretty voice, stuff like that. Don't have to press, you're not trying to get anywhere or anything, just build up some starter confidence in expressing yourself. Like the other guys said, if someone doesn't seem receptive, don't sweat it, just back off. Nothing wrong with a compliment.
It's a trial and error thing, though, and you'll develop your own style over time.