The first time you do any commentary, you will have trouble getting past a few minutes. Lets plays are especially hard, because it is hard to find things to talk about. I would start with gameplay commentaries until you are comfortable enough to move to lets plays. Also, lets plays tend to get very low views on channels without established subscriber bases, because they are very hard to optimize for searches. I have always just taken a few notes or just thought in my mind what I would talk about. Some commentators go as far as only recording around 30 seconds at a time of audio then put all the thoughts together. A good example of this is MinnesotaBurns in his "Hate Mail" series. Sometimes you can hear a mouse click or some weird pauses. That is where he stopped recording. If you write the whole thing down, it can start to sound very bland, so I would avoid that. Also, do not be afraid to start over. Sometimes your first take won't be your best work, and you will need to do it again.