First of all, you have to decide if you really want to actually be a reviewer, and if so, why? My motivation for starting down that path was that I really like trying stuff out, and I enjoy sharing my findings with people. But to do a good job reviewing takes a considerable amount of time and energy, and quite frankly, especially in the beginning, money. When I started reviewing stuff on head-fi, I had to buy everything I wanted to review. After quite a while of doing that, I started to get some manufacturers reaching out and offering to loan me gear to review. But that was after I had already written dozens of reviews of stuff I had bought in order to review it. And even now, some of the loaners I get I end up paying return shipping costs when I am done with them, although some manufacturers cover that. So there is even some ongoing cost when you get to the point of getting loaners.
Then there are some manufacturers who are not so nice to deal with. While most are great, I had one portable amp maker send me an email instructing me to change my review! When I refused, he backed off, but he had been pretty rude about it. Another threatened me when the he did not receive the review loaner back. Turned out the post office had attempted to deliver the item three times and then eventually returned it to me. Totally his fault.
And you better have a thick skin, too. I've had head-fiers call me all kind of names, publicly and privately, because I didn't like their favorite product. Want proof? Go back and read the thread of my review of the HD800, although the most egregious posts were of course removed by the mods.
So in short, make sure you REALLY want to be a reviewer before you set out to become one, because you will invest a bunch of time and money, so you need to be sure you want to do it. I still enjoy it...most of the time