Hi Bechara -
Can you give some more details? It can get a little bit complicated, not quite as bad as “How do you do brain surgery, please?”!
What kind of song are you importing? The choice of how to detect the song tempo will vary depending on whether it is a drum-type loop, or a full arrangement song such as off a CD.
If it’s the latter (like a full song off a CD), “tap tempo” might be a good thing to read up on in the manual, then “Time Warp”. Also, do you know about Musical Mode?
Others will be along soon to help out … but reading ahead of time is likely to help you understand the answers better.