Fixed Effects Regression

#1
I am working on a project that has a substantial degree of variability in the availability of data from the beginning to end of the study. I therefore need to make sure my findings aren't a result of changing sample composition. I need to run a fixed effects regression to find the "Within Firm Trend" similar to this: http://imgur.com/XMol719. Can anyone please advise me on how to do this in Stata.

Any help would be greatly appreciated.