Definitely keep a journal, maybe even start a vlog, even if you're the only one who watches it. Store the videos in DropBox and password-protect them if you need to.
If you keep a journal, I'd put down stats such as current heart rate, blood pressure [you can buy wrist cuffs just about anywhere for next to nothing], height, weight, mood, appetite, energy level, and make some general notes about how you feel. I read one guy's blog where he would write about his exercise regime sometimes, and in one entry he said that he noticed his muscle mass was increasing. He also took photographs of himself from month to month for a while. By the end of his first year on T, you can see a big difference in his entire body.