Also, how much memory would an hour long recording use?

I tried OBS but it just recorded a black screen.

I recently started speedrunning Bioshock. I wanted to record my game play so I could get on the leaderboard. I was wondering what recording software everyone else uses and what I should use. Is it possible to record an hour long video using free software? Do I have to buy some sort of capture card? If so what should I buy?

Thanks