Simply google "define popular": "liked, admired, or enjoyed by many people or by a particular person or group", whereas "define famous" will give you "known about by many people".
If famous and popular were the same thing, you could comfortably say "Adolf Hitler is a famous and still popular person" out loud ... ;-)
"This team is popular" = "Bu takım popüler". "This is a popular team" = "Bu popüler bir takım"
It might be helpful to think about where the "is" goes, since it doesn't show up in these types of sentences in Turkish. You can see that way that the subject and the predicate are different in both sentences. "This team is popular" = "Bu takım [is] popüler". "This is a popular team" = "Bu [is] popüler bir takım"
The "bir" is nearly always standing directly before the word it belongs to. You say "kırmızi bir gül", (a red rose), "iyi bir insan" (a good person). To say "bir kırmızı gül" would mean One red rose etc. To put the "bir" before the adjective would change the meaning.