You use the polite form of language when talking with people you meet for the first time, and with people who are older than you.
ちょっと長い文章になってしまいますが...
"You use the polite form of language"=「敬語をあなたは使います」
"when talking with people you meet for the first time"=「始めて会う人たちと話しているとき」
"and with people who are older than you"=「そしてあなたより年上の人たち(と話しているとき)」