Being courteous is the right idea if you want to be on friendly terms with others. The more courteous you are to someone (and vice versa), the more a sense of closeness grows between you.

懇意 formally describes being on good, cordial terms with someone. You might talk about a colleague you've become friendly with (懇意になった同僚) or a professor who treats you warmly (懇意にしてくれる教授), for example.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.