|Your didn't say whether your data was in rows or columns so I'll toss out a suggestion and you can rearrange it as you like.|
I am also assuming that you want to record/save your actual weight each time you way yourself:
In A1, enter the text Current Weight
In A2, enter your starting weight, e.g. 150
In B1, enter the text Total Lost
In B2, enter this formula:
=A2-OFFSET(A2, COUNTA(A3:A1000), 0)
In C1, enter the text Percent Lost
In C2, enter =B2/A2 and format the cell as a percentage
Each time you weigh yourself, enter it in Column A. First in A3, then in A4, then in A5, etc.
The formula in B2 will use the COUNTA function to count how many entries are in A3:A1000 and use that number as the rows argument for the OFFSET function.
The OFFSET function will grab the last value (weight) in the list and subtract it from the Starting Weight in A2.
The formula in C2 will calculate the Percentage lost.