You can not upload something that is copyrighted by someone else without their permission, that includes video game footage. There are exceptions to this like "Fair use", I'm not a lawyer so I won't get into that but you can look it up if you want.
Some publishers and developers are really good
(Riot Games, Mojang) about monetization, but some aren't
(Nintendo, Capcom)
If you decide to do a video just send them an email before hand asking if you can upload and monetize it, that's all there is to it. I've never done it so I don't know exactly what you would want to say, maybe someone else with a little more experience can help with that.
I have an article bookmarked about a guy who is contacting as many publishers as possible and asking if he can monetize videos of their games. He than post the results, it's an interesting read but I don't think the forum allows linking to thing like that
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Anyway... have any other questions?