If your Excel sheet is big then make up a smaller example sheet, something with perhaps half a dozen rows should be enough to show what it is you want to fit.
Then in Mathematica do the Import.
Select the In[1]:= and Out[1]= cells by clicking on the thin little cell bracket along the far right edge of the notebook. Copy that to the clipboard. Paste that into a reply here.
Then add a bit of text at the bottom of that reply explaining what kind of function you want to Fit to this, a straight line?, a parabola?, some fairly specific simple concrete function you want to fit.
With that it should be fairly simple for someone to take a moment and show you a line or two of code to do what you want.