I'm going to change up the notation you're using just to make it a bit more clear. Instead of M_0 for the mass at time zero, I'm going to use I for the initial mass.

The given decay equation is then

You've done a fine job with the logrithm to rearrange this into

The table lists the independent variable and the dependent variable. Your new table should do this as well. Thus, since your 'independent variable' is now you can list it in the new table (just as was done in the original data). Since your 'dependent variable' is you should take the natural log of each mass in the original table and list that result in the new table.

This sort of problem comes up quite often. Another way to get a linear graph of this data is to plottversusMon graph paper specifically designed for this kind of data (called semi-log paper), in which the y-axis has a logarithmic scale.