Flute trills are common in music written for flutists. Dainty flutterings of whole tones and half tones, these can be gentle undulations or jackhammer-like key smashing.
You may occasionally see a flat, sharp, or natural sign above the "tr" marking. In this case, the note being trilled to will raised or lowered a half step. The natural sign would remove the flat or sharp accidental that occurs in the key signature.
If the note is a quarter note, trill approximately at the speed of 16th notes.
In fast and furious passages, feel free to trill as fast as you can.
The turn may be marked with a sideways "S" or it may not--simply left to the flutist's knowledge of performance practice.
The sideways "S" may have flats, sharps, or natural signs above or below it, just as the trill mark can. Just as with the trill, play the note flatted, sharped, or natural, depending on the key signature.
There are many books available with flute trill fingerings and alternate fingerings, as well as some great on-line sources.