Timing is one of the hardest things to get perfect on a map
You basically just have to tap to the song on the timing panel and get the bpm as close as possible so that it follows the exact rhythm of the song (you can also try a bpm analyzer) but they are often far from perfect, it may give you an idea though
As for the offset you are meant to place it at the first beat of the song, so first go to the beginning of your mp3, put it at 25% or 50% time and just try and place it as close to the first beat as possible (headphones help sometimes)
Then as you start mapping continue to test and check to see on the left how your timing can be improved (how many milliseconds your hits are off by)
Or you can just ask someone
But its a nice thing to learn to do