Bara-'s definitely on the right point that a song properly timed should not be playing around with 1/8. If you're consistently on 1/8 snap, you may want to adjust your offset or BPM.
Actually, this may answer your question too – if you want to
change the BPM, as perhaps the song you're mapping changes BPM, you'd want to open the Timing Setup Panel and click on the + that says "Add Timing Section". Alternatively, you can go to the Timing toolbar and select "Add Timing Section" there, or Ctrl+P. You should only do this if your song has variable BPM, that is, if the song clearly indicates a speed change.
You probably just have a mistimed map, so you may want to take a look at the
osu!academy's Timing basics to get a good idea of what's in your toolbox.