As basic as it sounds, read guides/watch videos online on flirtation/signals that women give if they're interested etc.
Flirting is totally a skill you can practice, no one's perfect at it from the beginning.
If you're unsure about whether a girl is interested or not try to lightly test the waters, make some remarks that you could play of as jokes and such.
Like the other anon said try some light body contact, see how she reacts.
"That guy" usually implies just being way too forceful/not being able to read and interpret a girl's body language. You can usually tell quite easily if someone feels uncomfortable or not, just don't keep pushing anything if you can tell that she feels awkward or tries to change the topic or such.